Could you take a look at what Firefox thinks you were using before? Here's how:

Type or paste this shortcut path into the Windows search box, or the address bar of Windows File Explorer, then press Enter to submit it:

%APPDATA%\Mozilla\Firefox\Profiles

Each of the weirdly named folders here will have its own compatibility.ini file which lists the last-used version of Firefox. Do you see any anomalous results that might indicate a hidden installation of Firefox?

I followed the advice in this related support question , which was to delete all the Mozilla tasks in Task Manager. It's been three days and the profile error hasn't shown up since I deleted all the tasks in Task Manager.

I didn't think of that, just the scenario of starting Firefox normally. Glad you found it.