What do you mean by "check"? At the moment I have no idea morte what to do, check or try. I have reinstalled Firefox several times and done sone othe things that didnt help. The funny thing is that it worked 10 months no prob and it still works on other Macbooks but with mine nope...
Have you tried the Safe Mode? This would allow you to see, if any of extensions installed (for instance Astrill Proxy Switcher, mentioned above) is behind it. Safe Mode is a troubleshooting mode, which disables most add-ons.
(If you're not using it, switch to the Default theme.)
You can open Firefox 4.0+ in Safe Mode by holding the Shift key when you open the Firefox desktop or Start menu shortcut.
Or open the Help menu and click on the Restart with Add-ons Disabled... menu item while Firefox is running.
Once you get the pop-up, just select "'Start in Safe Mode"