I'm not sure who owns the WGA plugin for Firefox but its sheer laziness that the installer hasn't been fixed yet.

Easiest way around it is to just do it in Internet Explorer. With windows XP even if you uninstalled IE and physically ripped the folder out by the roots its regrew itself for the purposes of windows update. Click the "fix this problem link" that security essentials opens in the balloon (or open security essentials and find the button for it), that will open Firefox to a genuine advantage "action required" page. Then open IE (use the windows update shortcut if you uninstalled IE). Copy the address of the page that opened in Firefox into the address bar in IE and hit enter, it brings up the same page but this time it works correctly and suddenly security essentials is happy. Now if only I could figure out why it popped back up and i had to do it again a week or two later. It might be running the check regularly or with updates. But I suppose its easy enough to do every once in awhile until they bother to fix the plugin.