try to create a new firefox profile. See here [[Use the Profile Manager to create and remove Firefox profiles]]

Try to disable hardware acceleration in Firefox. *Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available" *https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes ---- You can check for problems with current Flash plugin versions and try these: *disable a possible RealPlayer Browser Record Plugin extension for Firefox and update the RealPlayer if installed *disable protected mode in the Flash plugin (Flash 11.3+ on Windows Vista and later) *disable hardware acceleration in the Flash plugin *http://kb.mozillazine.org/Flash#Troubleshooting