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Have you recently installed an add-on such as an extension or a toolbar? Some extensions suffer from memory leaks that can cause Firefox to crash.
Clear private data by pressing CTRL SHIFT DEL.
You can also run Firefox in safe mode - if it runs without crashing it suggests a bad extension or toolbar.
Close all instances of firefox (check task manager for firefox.exe and kill the process if necessary)
Now go to start>all programs>mozilla firefox>mozilla firefox (safe mode)

Also check for spyware as a precaution; use superantispyware if you don't have a spyware scanner - free here
http://www.filehippo.com/download_superantispyware/

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