You might be having an issue with a Firefox extension, check out this site:
http://kb.mozillazine.org/Problematic_extensions
I had a problem with FF stalling, slowing down and sometimes needing multiple refreshes to load a page.
The problem turned out to be the extension for Down Them All, which screwed up some of the HTTP settings. Unfortunately, uninstalling the extension didn't fix it and manual edits were needed (simple enough to do once you know how).
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