Tonight I finally got frustrated enough to look for a way to disable it. I tried searching on things like 'disable TinyMCE' and 'block TinyMCE' - but I just kept finding pages with instructions to web masters on how to incorporate TinyMCE and allowing their users to disable/enable it. I thought about trying to put something in userContent.css, but that was too much work.
It occurred to me that all I really needed to do was stop the TinyMCE JavaScript file from loading (yes, I know, turning of JS completely would do that - I use too many sites where JS is worth having, IMHO). How to do that? Well - how about abusing AdBlock Plus? Brilliant! I installed the AdBlock Plus extension in Firefox, and simply added a rule for '*/tiny_mce.js' (what the file is normally called) and it no longer loads. Mission accomplished.
So keep it in mind if you run into things like this - AdBlock Plus can block any content from loading, you just need to enter the right URL pattern. It is useful for more than ad blocking. :-)