Some people believe this problem is caused by the Flash plug-in, but I have not been able to verify this. To solve this problem, they're suggesting that you remove this plug-in. In my humble opinion this is not an option, too many web require this plug-in in order to use it.
To help prevent this problem from happening, you can try to make a change to Firefox's configuration. This change prevents the browser's cache from getting too big.
- In the Firefox location bar, type "about:config" and press the Enter key.
- Right-click anywhere in the browser windows, and select 'New>Integer'.
- In the Value dialog type "browser.cache.memory.capacity", press the OK button
- In the Value dialog, enter the maximum amount of memory you want to allow Firefox to use (i.e.: 65000), press the OK button.
- Restart the Firefox browser.