... Every time, it turned out to be an add-on. ...
This says nothing to the "too many variables" problem. I use multiple browsers and have only flirted with a handful of Firefox add-ons (other than the ones they push via updates, and those don't fit my definition of add-on no matter what the UI says).
I mostly use Opera these days, and if simple infrastructure glitches aren't to blame for the occasional forum burp, it's more likely that I'm 'undermining my web experience' by using Opera's ability to block 'content' from third-party domains like Google analytics.