Recently, it happened to me that Firefox started loading a completely white, blank page. Strangely, always the same pages remain empty while other pages load without problem.
My first hunch was a misbehaving Add-On or Greasemonkey script. Deactivated everything - no effect.
Then I began searching Google, mostly discovering Windows related threads with people tracking this down to a virus - on Linux, quite improbable.
The solution was, actually, that it seems Firefox has a small bug that breaks your user profile on upgrades. So start Firefox with the ProfileManager option and generate a new profile, or delete/move your old one in ~/.mozilla/firefox.
After that, Firefox will load pages again, and the blank pages are history.