Duncan this looks very annoying and I am sorry that this has prevented the ease of use of Firefox. When I run into this I usually blame javascript or my cache because something is not working correctly. If it is happening on many sites, I would first recommend safe mode to make sure its not hardware acceleration, a malicious add on or any changed Firefox settings. [[Troubleshoot Firefox issues using Safe Mode]] Second try a new profile to make sure its not an odd ball setting that safe mode does not discover:[[Use the Profile Manager to create and remove Firefox profiles]] If its just one site, clear the cache of that site or any old cookies:[[Delete cookies to remove the information websites have stored on your computer]]