My home page is 'Mozilla Firefox Start Page' and it is not Google but Yahoo. I don't want to change it google.com because the Mozilla one restores my previous session. I've tried going into about:config and reseting browser.startup.homepage_override.mstone still does not work.
You can always restore the previous session via the (Firefox >) History menu.
Is this the search engine on the about:home page or is the home page set to the Yahoo site?
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.