Firefox freezes at random times, forcing app closing/restarts, or system reboots.

[rado84]

Therein is a confusing point: I removed all extensions earlier - however, upon reopening the instance, I found that some extensions were reinstalled. I removed them, and reopened the application from the terminal to test; I'll update with results from there, while I experiment with memtester.

Yeah, firefox does that, if you have used "Recommended extensions" before - you remove them but they come back. One of the reasons I abandoned this browser.

 

[Condobloke]

But without knowing the desktop and its version, I can't tell for sure. I'd suggest first start with a clean profile. Delete your current profile (I think it's in ~/.config/firefox or ~/.config/mozilla by their new standards) and redo your preferences manually one by one. Don't install extensions at first to see how it goes.

I believe the problems still exist in the profile being used. @rado84's advice is well worth following.