1) Try running firefox in safe mode.
http://support.mozilla.com/kb/Safe+Mode
(This turns off your add-ons and other customisations, so just try browsing as you normally do and see if the same thing happens.)
If you can do what you normally do fine, then it could be an Add-On causing troubles. Start firefox as normal, turn off all Add-Ons, and then restart and turn on each one, one-by-one. Hopefully you'll notice if things start going wrong and can pinpoint the Add-On.
More instructions here:
http://support.mozilla.com/kb/Basic+Troublesho oting