If you use extensions (Tools > Add-ons > Extensions) like Adblock Plus or NoScript or Flash Block that can block content then make sure that such extensions aren't blocking content.
Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).
Do NOT click the Reset button on the Safe mode start window or otherwise make changes.
I had a problem with Ebay pages leaving large white spaces. Safe mode worked correctly so I had to disable all my add-ons and then re-enable them one at a time.
Turned out it was "NoScript ver. 220.127.116.11" . Haven't tried an update yet.