Firefox - always in front

Annoying and now frequent bug nagging me on my Mac: my Firefox browser window suddenly decides it always deserves to be in front, no matter what. The only way to get move it out of the way is to minimize it or close it. Does anyone have a fix for that?

Plugins like Firebug and the Web Developer Toolbar make it the browser of choice for work, but shoving its way to the front at all times makes Firefox a rather rude houseguest.

Firefox, Mac OS X 10.5.1