Came across a problem with Firefox 4 and 5 today. It is a rather esoteric problem but it is a pain in the arse if you get it. I upgraded to FF4 last week and as soon as I did Quickr stopped working. When I went to the quickr home page I got 6 errors logged in Firebug starting with an "unterminated string error". this only seemed to be affecting me, the tech support chappies were all OK with both Firefox 4 and 5.
I started disabling plugins and addons and it was when I disabled the Lotuslive chat extension that the problem went away. Rather than just blame the Lotus Live extension I uninstalled FF and all the add-ons and extensions and installed a clean version 5.0.1 with no add-ons and re-added Firebug, Flashbug and the LotusLive chat extension (in that order) and Quickr continuted to work normally, so I would guess that it was a more complex issue that just the Lotuslive chat extension that caused the problem.
So if you come across Firefox not working for quickr and you get 6 errors starting with an "Unterminated Sting" error I suggest you uninstall FF and then do a clean install and everything should be fine