This seems to be a problem that has started since Firefox 7. See the instructions in this article for a possible solution: Firefox never finishes loading certain websites

I might have discovered the solution without starting over with all your places and bookmarks.

Right after i installed 7.0.1 over a week ago,every 10-30 min When i would be watching videos online, it would freeze for 20-30 seconds. Firefox would say ( Not responding) both firefox and the plugin container process would be very large memory files. I tried to go back to Firefox 5.0. No avail it would still freeze.I tried to disable the plugin container . no good. I tried to adjust memory or cache or plugin settings in “about:config”.Nothing fixed it!

I finally discovered why my Firefox 7.01 was periodic hanging fix. It is something to do with some of your personal files. download this this add on and install it https://addons.mozilla.org/en-US/firefox/addon/places-maintenance/

or you can Google "places maintenance Firefox"

After you install the add on, restart Firefox.go to "tools", then "add ons" and then click “extensions” tab ,then click' options" on the "places maintenance" entry. choose "ALL' or just "expire" and hit "execute", then wait until its done., restart Firefox. should be fine then and all my bookmarks, customizations ,settings, and history stays untouched!

After i did this my Firefox did not freeze once for over 3 days straight. Might not work for everyone. I already read that Mozilla is aware of the problem and are working on a fix it in the full version of 8( the 8 beta did not fix the problem for me) . But they said this is a good temporary fix at least,( might be permanent)

For me at least ,Firefox works fine in 7 now. perfect actually.And many people i saw on similar forums had similar problems.

Enjoy. Hope this helped