I have this too and am hoping for answers here at Firefox help

I think I found it and I've fixed it on my computer. In Add-ons do you have an add on called Game Play Labs? If so, unistall it. I had 2 different ones that said Game Play Labs. I unistalled both and haven't had the red banner warning pop up at all.

I found the info by googling.

i had the same problem and its gone after i manually updated to firefox 4....also i did what ceebee said and its working just fine.

OK nobody is telling how to fix this and Ive found only one way If you DO NOT HAVE the game play addon in the addon directory , first back up your book marks so you don't have to worry save as HTML file on desktop , next and this is the part nobody want to admit you have to allow then install the add on restart fire then go to Addons and disable the game-play labs Add on restart and GONE! if that doesn't work did for me uninstall reload and put back the bookmarks

If you were not dumb enough to "allow" the installation of a suspicious plug-in, you can still get rid of the red banner and prompt to install your "critical update" by looking under the Extensions tab of your Add-ons. Game Play Labs hides this bit of malicious code as a Video CODEC.