it is a similar problem to the new tab/window problem i reported. In fact if you try any of the items on the right-click menu, you will find almost all of the items do not work.! only "select all" and "inspect element with firebug" work on my windows pcs.

disable/uninstall firebug extension and firefox will start behaving again.

Why should I disabled the firebug? Whose defect is it? Firefox or Firebug? Either Firefox or Firebug, one of them should be fixed. I hope they release the next version soon with this bug fixed.

I have found that uninstalling firebug, restarting firefox, and reinstalling firebug seems to fix the problem. If that works for you, please mark your question soved :-)

-update, you should not have to reinstall firebug. just closing and restarting firefox should fix the problem.