We have migrated our site to EPiServer CMS 7.5. The TinyMCE toolbar seemed to be missing in our pages with XHtmlString property. Do we need to download our own version of TinyMCE?
Can you see if you have any 404 calls to any Tiny MCE plugins or other script exceptions?
It seems that you are missing a custom Tiny MCE plug-in which seems to be added to the configuration. If Tiny MCE cannot load this plug-in, the initialization fails in Firefox. Add the missing script file and it should probably work.