It seems Firefox doesn't really rely on these. My profile directory has been around for more than a decade, went through three computers and even between Windows and Linux and from plain Firefox to Firefox Developer Edition and pretty much everything transferred just by copying the files around (however i didn't copy the full Mozilla directory, first i let Firefox make a new empty profile by itself and copy/pasted the files in it, overwriting whatever was already there).

It even had the original XUL-based DownThemAll version, got disabled after XUL addons were disabled and some time one or two years later it got re-enabled again after the dev released a webextensions compatible version (sadly with several limitations, but still useful for bulk downloads).

Amusingly, there are a couple Windows absolute paths in there even though this profile has been on Linux for a few years now :-P

This is very important to know if this is really the case. And if it is, then what is the best way to migrate? Is there an official, supported method if "mv" is not it?