hello, maybe that's an issue with hardware acceleration - please try updating your graphics driver, or in case this doesn't solve the issue or there is no new version available at the moment, disable hardware acceleration in the firefox menu ≡ > options > advanced > general (that setting will take a restart of the browser to take effect).

Skipper2 said

Followed Firefox reboot (x2) after problem with Norton.

Notice that the Firefox symbol has a windows blue/yellow shield on it.