Firefox scroll-bars disappeared (after add-in)
 

Not quite a straight-Linux question here but ... the scroll-bars at the side of my Firefox browser window disappeared, I think after installing an add-in.

Any ideas as to why? I mean, it's not an earth-shaking problem but what t'heck causes it?

Extensions can sometimes be buggy, especially in certain combinations. Just this morning several of my extensions went through auto-updates, and when I restarted the browser just about all of my toobar icons had up and disappeared. I'm guessing it was some kind of conflict between Tab Mix Plus and Toolbar Buttons. I finally had to download and revert to a previous version of Tab Mix Plus in order to get them back.

So, assuming you don't know which extension is causing it, start by disabling extensions one by one (and restarting between each one), and hopefully you'll figure out which one is causing your problem. If not, I suggest wiping your profile and starting over.