There are so many different Macs/Mac OS X versions along with Firefox versions about & fixes aren't usually "one size fits all" so it's crucial to supply the above info before anyone can help you.
We just need your version of Firefox as honestone requested.
Then someone will hopefully be able to sort your issue out.
Well, given that you are using OS 10.8, that version of Firefox that you have is the last "unsupported" version available. This article reiterates that:
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