I'm just passing along a fix I came up with. I had numerous problems with the admin of 2.8.4 after upgrade. Quite a few things weren't working, I could edit my posts/pages, TinyMCE was not working at all. Couldn't click to the HTML tab, my edit box screen was blank, Screen Options wouldn't drop down, couldn't click on quite a few things, if I did nothing happened. That's a quick summary. I finally had to use Opera to do things in the admin and that had it's own separate case of the Glitches, so I wasn't happy with that.
I am a Firefox user and normally use v2x. Out of desperation, I flipped on my laptop which has Firefox 3 and Internet Explorer 7 and found I was able to work fine in both browsers on the laptop. I then grabbed a copy of Firefox 3.5 and installed that and now I can work in my admin just fine (so far but I just started so no guarantees).
I really didn't want to upgrade Firefox, heard Firefox 3 has some issues I didn't wanna deal with ... but we'll see I guess.
Hope that helps someone.