hello, can you try to replicate this behaviour when you launch firefox in safe mode once? if not, maybe it's an issue with hardware acceleration or a theme...

Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Ok, turning off hardware acceleration fixed it. The odd thing is this problem only occurred recently, and there has been no change in hardware.

another thing you could try is updating your graphics driver & see if it's then working with HWA enabled again.

this should be the right link for your model & system: http://support.amd.com/us/gpudownload/windows/Pages/radeonaiw_vista64.aspx