I recently found out that Firefox has a feature called “prefetching” that tries to pre-download items that it suspects you might click on soon. This could help in pre-downloading content that you would visit next (ie; it is linked on the page you are visiting therefore you might access it next), but it can also have the nasty negative effect of wasting your bandwidth on items you don’t ever want. This can also download cookies from sites you haven’t visited, etc. Seems like a nasty feature to me!
I also found that this prefetching feature will cause your connection to the ubuntuforums.org (and a few other sites) to be refused if you are also going through a squid proxy. So, this is a quick suggestion on how you can disable prefetching in Firefox.