multiple plugins running in firefox causes hang and glitches

I can get up to 20 flashplugins running which realy cripples the browser. The graphical glitch is like another copy of the browwer laid on top a bit higher.

I have tried removng hardware acceleration and theseciriy stuff but it still happens.

I've updated the version several time but it still occurson 11.9.900.117.

Windows 7 64 bit with Nivdia GTX670.