If you have added server side plug-ins you need to add an empty script file for the plug-in, otherwise at least Firefox will choke. See the net traffic for any 404:s and add an empty JavaScript file. In EPiServer 7 you can set a Tiny MCE plug-in to be "server only".

Tinymce uses script tags to load plugins in parallel. As I observed, on Firefox and Chrome, once a script tag failed to load, onerror event would make the others stopped as well. IE is suprisingly more fail tollerent.

Am going to try the suggestion, but interestingly, in version 6, the Firefox error with the 404 happens, but the WYSIWYG works.

Yes. Something happened in the Tiny MCE version we upgraded to in EPiServer 6 R2 that makes this fail for some browsers.

Is there a fix for it?

There is not a hotfix for is that is what you meant but the solution is simple, as described above: Just add an empty file to be loaded. In EPiServer 7 you can mark your plug-in as server side only and then we won't add it to the list of plug-ins that Tiny will initialize.