From CNET:
Mozilla blocks Skype's Firefox-crashing add-on | Deep Tech - CNET News

The extension has been the No. 1 or No. 2 cause of crashes for the current stable version of Firefox, according to comments in Mozilla's bug tracker. And the plug-in dramatically slows Firefox's processing of Web page elements through what's called the Document Object Model (DOM)--by a factor of 3 to 8 with a newer 5.x version and by a factor of 325 with the older 4.x version, Mozilla programmer Boris Zbarsky said. The effect of this is to make pages appear to load much more slowly.
Earlier in January, a Skype representative acknowledged that the company knows about the issue. "Look out for an update in the near future," the representative said.

Has this happened to you? It has to me, so I've been forced to use Google Chrome vice bouncing back and forth between the two.