I use TinyMCE as the HTML editor for one of the projects I work on.
While using the TinyMCE editor today I found that the stylesheet drop-down did not list any styles in Firefox. The styles were showing up correctly in IE.
I disabled all the plugins. And TinyMCE styles worked as expected!
After a bit of googling, I found that it is the Skype plugin for Firefox that causes issues with the stylesheet drop-down in TinyMCE.
I now officially have a love-hate relationship with Firefox.