I can't seem to edit the post so additional information in here.

It actually seems that only the default profile is reset. My other profile was actually intact.

It seems that the best way to keep ones profile functional is to leave the initially created default profile unused and create another profile where to store all the user data.

I seem to have been wrong assuming that his has anything to do with the update process.

Instead there seems to be something wonky going on with the Profiles in general. I was using today my secondary profile when I opened my main profile from the top bar. The main profile opened correctly with the themes and extensions it was supposed to have.

However after closing the window running the main profile and then re-opening it later I noticed it had no theme and none of the extensions I had earlier installed. It was also disconnected from my Mozilla account. The secondary profile didn't have most of the extensions that the main profile had and it was not logged in to my Mozilla account. Only the main was logged in.

Is there a way to verify if the profiles are somehow overwriting each others files partially?

You can find "Edit this question" under the sidebar "Question Tools" menu (you may have to scroll to the top). For a reply, "Edit this post" is in the three-dot menu next to a reply.

Firefox would normally store its main profile in "~/.mozilla" and the secondary profile in "~/.cache" Profile management is done via profiles.ini and installs.ini in "~/.mozilla/firefox". Are the profiles gone in these locations or is this a problem with profiles.ini?

• https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible

You can find "Edit this question" under the sidebar "Question Tools" menu (you may have to scroll to the top). For a reply, "Edit this post" is in the three-dot menu next to a reply.

Oh, haha - I was expecting it to be under the three dots for the post as well that I didn't see the button on the side... Thanks.

Firefox would normally store its main profile in "~/.mozilla" and the secondary profile in "~/.cache" Profile management is done via profiles.ini and installs.ini in "~/.mozilla/firefox". Are the profiles gone in these locations or is this a problem with profiles.ini?

• https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible

The brief response is that there is only "extensions" in the "~/.mozilla/" directory. No "firefox" folder at all. Instead I can find all the (assumedly) profile directories under "~/.config/mozilla/firefox" along with the profiles.ini and installs.ini.

In the profiles.ini (the one existing under ~/.config/mozilla/firefox)there are Profile1, Profile0 and General tags. I'm not completely certain how to read the entries, since the profile selector shows still 3 profiles for me.

I ended up doing quite a bit of retesting and updated the question accordingly. But I'm hitting a bit of a wall, so I'm happy that the issue seemingly only wipes the pre-created profile.