I appreciate the time you guys have taken to respond to questions and complaints about this, especially @Gijs. I still think some aspects of this plan should be revisited or expanded, including the commitment to not maintaining an option to restore the unknown content type dialog. I'm sure this comment is gonna be nightmarishly long so I apologize in advance for the word vomit, but it's a complicated issue and I want to make the best possible argument for preserving the download functionality as I (and many others) currently use it.
I'm pretty sure I understand the motivations for getting rid of the tmp folder behavior, but I believe removing the UCT dialog severely restricts user options, particularly with regard to save location and choosing file handlers. It either restricts your per-download options or it requires you to choose a save folder individually for everything you download. Just consider that you may not always want to open a file of a given type in its default handler. I download SVG files and PDF files a lot. Sometimes I want to view them in firefox; sometimes I want to edit them in VS Code or Acrobat or w/e. If I set them to open in the default system app or to open in firefox, what do I have to do if I want to deviate from that behavior for a specific file?
I need to 1) close the file that just opened automatically; 2) open the downloads panel; 3) right click the download item; 4) click the menu item to open it in the system viewer. This menu item doesn't even give me the option to choose a different app, by the way. Only the default. So it's not remotely a 1:1 substitute for the unknown content type dialog in the first place. But even if it was, this is much more onerous than what I have to do currently:
Right now I have it set up (with the temporary pref
browser.download.improvements_to_download_panel) so that when I download an SVG file, it doesn't immediately open in Firefox. But "open in Nightly" is the default selected radio button in the dialog. (or at least it is on sites that aren't currently broken by bug 1747194) So if I want to do the default action, I only need to hit enter or click the primary button. If I want to deviate from the default action, all I need to do is click a different radio button and so on.
Moreover, when I want to save a file without opening it, generally speaking I want to choose where it's saved. But if I just want to quickly open a file, (especially if I want to open it in firefox) then I don't want to worry about where it's saved. (Frankly, I don't even want it to be permanently saved in the first place. I'd rather just pretend it never saved in the first place, almost like I opened it in a browser tab over http rather than file protocol. I get that I can choose my download folder, but that's not a replacement for the tmp folder, since even if I configured firefox to download files to my actual tmp folder, I'm still losing the automatic garbage collection. I'm still gonna need to clean stuff up by hand)
I get that there are arguments against the tmp folder, which is why I mentioned it last. I don't feel quite as strongly about it but I feel like losing the option is still a serious loss in terms of impact on users. I think many users will find it burdensome and take this as another affront. The justifications seem a little obscure to me, too, but I'm no expert. I don't doubt that they are real issues, they just seem to me to be outweighed by the compelling interest in temporarily saving files. Anyway, more serious is the lack of control over whether a particular file (not file type or disposition but just an individual downloading file) is opened.
The design doc has a strange comment on this issue: "If you want to open a file, you can now click the download item before it has finished, and Nightly will open the file immediately once it has." — Maybe I misunderstand the point of this, but the vast majority of my downloads are finished in a matter of milliseconds. Of course, if I'm downloading something giant enough that I have time to click the downloads toolbar button and click the download item before the download finishes, then chances are it's not the kind of tiny little file you wanna temporarily save just to quickly open and review it. But I don't think it matters, since whether you click the download item before or after it finishes, this still doesn't get you out of needing to choose where it's saved. So again, you either use the same folder every time for every file type in every context... or you have to choose the folder individually every time for every file type in every context, even if you're just trying to preview a tiny little file rather than "save" it.
I don't want to worry about where a file is saved when I'm immediately opening it. That is, I have firefox set to "always ask you where to save files." Currently if I choose to open in Nightly, it doesn't open the save as dialog at all, since it automatically saves in the tmp folder. With the tmp folder removed, there's this knock-on effect of losing that ability to not "save as" a file. Like, I want to have control over where I save files, but I want to be able to abdicate that responsibility at will, if I choose to. That's what I can currently do, and I do it all the time. If I'm losing the tmp folder, then at least it would be nice to be able to press a button to use a default save folder (just for this one time) and not have to deal with the "save as" dialog. But that can't really happen in the first place because there won't be any download dialog. The very first affordance will be the "save as" dialog.
I feel like these design choices are totally oriented around the default settings and behavior — save to "Downloads" folder without any prompt, don't open the file. From that POV I think this plan and the implementation so far make a lot of sense. If I'm content with everything I download piling up and collecting dust in the Downloads folder, then the downloads panel can become a kind of convoluted substitute for the unknown content type dialog. From there, I can at least open the file and dictate future behavior. And from that perspective I can see why you guys thought it was a great idea to make the downloads panel automatically open when a download finishes, so the user can immediately choose what to do with the file... basically like what the unknown content type dialog does.
In other words, it seems like in opening the panel automatically, we're trying to fill the giant void left behind by the removal of the unknown content type dialog. But clearly some people are irritated by the panel automatically opening, since a bug was opened for that. And it doesn't let you choose where the file goes anyway. It doesn't let you cancel the download, doesn't let you delete the file either. If I want to delete the file, e.g. since firefox is no longer gonna clean things up on its own, I'm gonna have to find it in file explorer/finder/whatever and manually delete it. If we're seriously committed to this course of action we should at least have a context menu item for deleting the file from disk.
So, the best I can do to replicate my current behavior is to simply accept that my "Downloads" folder is now a giant dumping ground, and revert everything to default settings. That way I can "choose" what happens when I download a file by opening the downloads panel. But that leaves me with no option to save the file to a particular folder. If I want that level of control, then now I need to accept the burden of choosing a download folder for every download, whether it's a file I need on disk long-term or just an email attachment I want to read for 30 seconds and never deal with again. Whereas now, I only need to make that decision for files I explicitly choose to 'save as'. I hope I'm articulating this well enough, it's kind of hard to explain what bugs me about this since it's basically a large "decision tree" that seems to have been severely pruned.
I reviewed the design doc but I still don't really understand if there are plans to recreate the same level of functionality I currently have, where with a single click I can choose between opening in firefox, opening in default system handler (VS Code if the file type is unknown), or save as.
Spending time individually binding specific filetypes to specific handlers can solve some problems, but I can't anticipate every file type I'll ever download. Many files I download don't even have a file extension, and I don't always want to do the same thing with every kind of extensionless utf/ascii file. I want to choose at the time I download the file, without having to deal with the "save as" dialog if I'm only trying to preview an SVG file or something.
Downloading files is one of the most important things users do with a web browser. Having to go through all these extra steps seems like a problem. I get that there are downsides but these are marginal in comparison to the benefits. Even the issue of saving to the tmp folder is silly in my mind, and that's the one I feel least strongly about. The possibility of causing odd problems in obscure environments/configurations seems obscure compared to the difficulties created by removing the unknown content type dialog. The fact that the tmp folder is harder to find is pretty marginal, since nothing is going to save there in the first place unless the user doesn't want to save the file permanently in the first place. After all, why would a user go looking for a file that they intentionally, explicitly chose not to save?
There's usually a hostile response to polemical comments alluding to the decline in users. But even if it's melodramatic, I think when people go out of their way to make comments like these it reflects something that should be taken seriously. I think most of us Firefox users are using Firefox precisely because we aren't content with Edge, Chrome, or Safari. If we wanted the behavior of those browsers, we would use those browsers instead. That's not to say they don't do anything objectively right that Firefox should imitate. But I'm just saying consistency with other browsers is not an absolute value or an end in itself. It's not really a value at all, imo. We shouldn't do anything simply for the sake of consistency. Rather, if Chrome does something that proves beneficial to the UX, we should imitate it not for the sake of consistency but for the sake of improving the UX. That choice would be equally worthwhile if the enhancement was thought up by some random bugzilla user rather than trailblazed by Chrome developers.
I also think it's worth considering adding a third option (in about:preferences) for where to save downloads. Right now you can only choose a single folder, or choose to "save as" every time. I think 1) this setting should be per-filetype, and 2) there should be a third option that means "always ask you where to save files if the default action for the file is to save without opening; but automatically save to your default download folder if the default action is to open the file with any handler." That would go a long way toward obviating the UCT dialog. But I really don't see a total replacement for the dialog, since you still don't get to choose individually whether to save the file with location choice; or open the file after saving to default location. You're relying on handler bindings, which are usually fine but some of us download really obscure filetypes all the time.