You can check the ssl and tls prefs on the about:config page.
If any ssl or tls pref is bold (user set) then right-click that pref and choose "Reset" to reset the pref to the default value.
Paste this regular expression in the Filter bar at the top of the about:config page:
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.