I get that alot too.I think Firefox just times out if it has to wait too long for a response from a web site.Not fully sure though.Someone else will explain it better.I think u can get some extension to fix it.
yeah first sun a virus scan and then run adaware to see if there are an y unwanted processes running...i used to get that error but only when i used certain programs .. do a speed check on dslreports.com