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.
Dear cor-el, thank you for your really helpful notes. And the Font solution worked!
but how can I find out what fonts have problem on my system in order to fix them?I want to see pages with their original fonts as before! (my loving google appears different)