As mentioned above, we aren't talking about Firefox's update mechanism here, but rather Librewolf's.
> Mozilla has been repeatedly resetting "Always check if Firefox is your default browser" option to "yes" with upgrades.
I'm sorry to say this, but this just seems to be misinformation.
I don't see that anywhere in the source code[1]? Anything I can find regarding prompting the user regarding the default browser is hidden behind an if guard to make sure the pref is `true` and not `false`.
The only scenarios I am aware of that will change the pref if the user has toggled one manually is the `_migrationUI`[2] function (as you can see, no changes relating to `browser.shell.checkDefaultBrowser`). Otherwise, untoggled prefs will be changed if the value in `firefox.js`[3] or `all.js`[4] is changed. As you can see, the last time the pref was modified was 2004.
[1] https://searchfox.org/mozilla-central/search?q=browser.shell...
[2] https://searchfox.org/mozilla-central/source/browser/compone...
[3] https://searchfox.org/mozilla-central/diff/94ff451885bb94679...
[4] https://searchfox.org/mozilla-central/source/modules/libpref...