I use XStandard to create rich content on my and my clients' sites. For those who don't know what it is, XStandard (www.xstandard.com) is a WYSIWYG editor that outputs XHtml compliant code. It's a really amazing editor and it does a fantastic job. The only problem I had was getting the CSS (it renderes CSS quite well in its ActiveX object) to link properly in FireFox browsers. It was working fine in IE browsers, so of course, it was hard to debug.
I finally figured it out. Basically, my CSS file is actually a CFM file that creates the required styles dynamically. You have to log in to use the Admin (where this tool is located) and the CFM (css) file is in this Admin. The problem was that FireFox was requesting the CSS file and for some reason, it appeared to the browser that it was not logged in. I don't know why IE did this fine.
So anyway, the end result was that I just had to remove security checks for that one CFM (css) file and all is well. If any of you create rich content areas, I highly suggest taking a look as XStandard. It's without a doubt the best editor I have seen to-date.
If XS uses Windows API (or IE com objects, its been awhile since I read up on all that) for accessing resources, it would make sense that logging in with firefox wouldn't necessarily allow XS to use that same context.
Did you ever try logging in using with IE, then switching over to FF while your IE session was still active?