(image upload somehow didn't work...)

Nevermind, fixed it myself.

user_pref("privacy.resistFingerprinting.letterboxing", true) was the culprit.

However, during my testing, Firefox somehow deemed it approriate to 'coerce' me into creating a new profile, because otherwise it wouldn't startup. Luckily, I had backed up some data beforehand, because I was kind of suspecting something like this would happen ( https://blog.fefe.de/?ts=a24001c4 ). BUT what the actual f*ck Mozilla? Why do you need to """improve""" Firefox with """features""" like this? And no, I don't want to use your botnet sync service! The same is true for Pocket and "Send" or whatever that crap is called, which, by the way, has absolutely nothing to do with a web browser. There's other (and better) software for that, you don't need to reinvent the wheel, especially when you're whining that bad bad Google is overtaking the browser market (which it quite frankly is, I'd have to agree), but what are YOU doing to 'combat' this matter which you are describing as so urgent? Releasing software that no one really asked for (and frankly for which there are better alternatives)?! I just don't get it...

Whatever, this is just an internet forum...

Thanks anyways, I was probably a bit too harsh, lol.