hello bsl55, probably that's an issue with hardware acceleration - please try [[Upgrade your graphics drivers to use hardware acceleration and WebGL|updating your graphics driver]]. or in case this doesn't solve the issue or there is no new version available at the moment, start firefox into safemode '''by pressing the shift key while the application is launching''' & disable hardware acceleration in the firefox ''menu ≡ > options > advanced > general'' (that setting will take a restart of the browser to take effect).