First, check to make sure you're using the most recent version of Firefox. If so, follow these steps: 1. Boot into safe mode (press F8 at startup)
2. Delete EVERYTHING in the following folders:
C:\Documents and Settings\user_name\Local Settings\Temp\
C:\Documents and Settings\user_name\Local Settings\Temporary Internet Files\
3. Empty your recycle bin
Boot back into normal mode and try using Firefox now.
Alternatively you may want to run a virus / spyware scan of your computer.
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What?