This is a wide phenomenon. It's the Flash player update that broke firefox. I can now crash FF with almost 100% certainty..The bug is happening when you close a page (or navigate to new link...) which has flash content in it while keeping another tab open at the same time, of course with flash content in that also.. So it's two flash tabs and you close one of them...
I'll try to check if i can put FF13 in to "legacy" state, where the plugin container is deactivated. Worked with FF8-12, noticed then that the plugin container brought more issues than what it resolves, sadly the settings are defaulted on each update (FF13, seriously? What, from v3 to v13 in a year).. It was created exactly so that if one tab (and it's plugin) crashes, it doesn't bring the whole program down but instead it clogs your system big time.. FF can easily take 1gb of memory when faulty plugin containers refuses to shut down. Or that was the case with FF12 and earlier.
EDIT: ok, i had conflicts in the config files.. Some parameters had been changed, some removed some added. Defaulted most of the and enabled the plugin container, let's see if it works now.