Could you try disabling graphics hardware acceleration? Since this feature was added to Firefox, it has gradually improved, but there still are a few glitches.

You might need to restart Firefox in order for this to take effect, so save all work first (e.g., mail you are composing, online documents you're editing, etc.).

Edit > Preferences > Advanced

On the "General" mini-tab, uncheck the box for "Use hardware acceleration when available"

Unchecking the "Use hardware acceleration when available" and restarting did not change the outcome. I also browsed the about:config options to see if there were any other related graphics options, but nothing else seemed to fit.

You could try updating your plugin to 11.2.

Apparently there's no native graphics acceleration for the Flash plugin on Linux (Adobe Forums: FP11 Linux Hardware Acceleration?).

I think there is a way to specify whether a specific plugin runs in Firefox.exe or plugin-container.exe, but my memory is hazy on that point. I think the relevant preferences in about:config start with dom.ipc.plugins but I'm not aware of an authoritative current source for how they work.