Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes). See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

If it does work in Safe-mode then disable all your extensions and then try to find which is causing it by enabling one at a time until the problem reappears. You can use "Disable all add-ons" on the Safe mode start window. You have to close and restart Firefox after each change via "File > Exit" (on Mac: "Firefox > Quit")

i have the same problem too. this happen last 3-4 days only in facebook ,while i am trying to play some games like mafia wars. -when i open first time the game and open many tabs (sending many gifts at the same time <---this work perfect since 3-4 days ago) at this first time i can close alla tabs

-but when i m trying to do the second time (send gifts to another friend with the same method) i cant close no one tab...i can close only the window with all tabs

-i did not something diferent to mozilla , i didnt config anything i have the same addons for years , (only 3)

i think it happen only in facebook... i have find no solution yet

Problems on Facebook can be caused by Facebook gadgets. You can try to remove them one by one to identify the culprit.