Not sure how to use that workaround. As a console command it says browser is not defined.
Anyway I can confirm this has still not been fixed. I tried just directly editing my extensions' manifest.json files to remove the "suggested-key" values. But firefox deletes the extensions on boot when I modify them, so I have to install them directly from an xpi file on my desktop, which makes them ineligible for online updates. What I really need is a way to change the local preference for the keyboard shortcut, not to change the extension's default shortcut. I've been trying to find a way to change the shortcut without modifying the actual extension file but I can't get the console command to work.
Anyway if you don't mind not receiving automatic updates for the extension in question, and you don't mind manually repeating this process for every update, then you can remove the default shortcut. Go to the profiles folder under AppData/Roaming/etc., find your profile, go to extensions folder. Locate the extension in question. If the extensions have random gibberish names, then open the extensions folder in 7-zip file manager and double click the xpi files. They are just zip files with a different extension name. The manifest.json is the file you need to edit and it also typically contains the title of the extension. If it doesn't, there are probably some other files in the xpi with clues as to the extension's identity. Icons and stuff. Anyway, open the manifest.json file, and find "commands". This section should have some of these:
"description": "Do the thing"
Just delete the "suggested_key" section entirely, so it becomes:
"description": "Do the thing"
This way the object and description will still exist in the keyboard shortcuts menu, so you can still use it. But instead of it giving you a default hotkey that you can't get rid of, it just starts out blank. I'm not sure what to do if you accidentally fill the shortcut yourself after that. Since I don't think doing so within firefox would edit the manifest.json file, yet I can't find any local, profile-exclusive file that contains user-modified shortcuts, you might just have to uninstall the extension, rename the extension "title" in the manifest.json or rename the XPI, or both, then reinstall it so that firefox doesn't realize you have preexisting settings for the extension.
Anyway after that, save the manifest.json and repack all the files into a zip, then rename it .xpi, then put it on your desktop or something. Then open FF, uninstall the regular version of the extension, and on about:addons click the gear and "install add-on from file" or whatever. Locate your XPI and it should install your version of the extension. May or may not receive auto-updates, if it does then there's probably a reference to the online extension ID (via the store page) written in the XPI that you need to remove, so firefox stops recognizing it as a legit extension. Obviously to do this I think you need to enable unsigned extensions which I don't think you can do in the main or beta builds. But if you're reading this thread in the first place you probably already get all that.