Being the default browser on a user's system only really matters when the user it doing something in other apps / parts of the OS that cause things to be opened in the default browser. (EG, clicking a link in an email app, or opening a local .html file). We have some indications that being the default browser may not have a large impact on usage, which would imply that users are not opening links/files from outside the browser as much as they have in the past. Some telemetry to record these instances would help us confirm that's the case, and understand modern browser usage. I haven't thought about it deeply, but I think the minimal 2 bits of data to record would be: * Was the current Firefox session started by launching as a default handler? * Post-startup, has this Firefox session been used to open something as a default handler? I could also imagine these being counters instead of flags. (Can we tell the difference between Firefox being launched as a default handler, and the user manually invoking it w/ a url from the command line? I'd suspect not, but also would suspect that such usage isn't common enough to matter.)
Interesting as well, though possibly tangential to comment #0, would be to know how the session was started (start menu, taskbar, desktop shortcut). We could probably do this by adding an extra argument to the executable based on where the installer creates the shortcut. Shortcuts created by the installer on the desktop could have 'firefox.exe --origin desktop', taskbar having 'firefox.exe --origin taskbar', and startmenu having 'firefox.exe --origin startmenu' (if this last one is possible). User created shortcuts and auto-generated start menu items would be missing these extra origin annotations. If this number was significantly high, we could add additional code to Firefox to count any user-created desktop shortcuts and possibly infer some more data from that.
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