Firefox doesn't handle MHT files by default, which MHT add-on do you have installed?
Is it still enabled?

http://en.wikipedia.org/wiki/MHTML
Un MHT - https://addons.mozilla.org/en-US/firefox/addon/8051
MAFF - http://maf.mozdev.org/index.html

UnMHT 5.7.0 is installed and enabled. It was working fine with Firefox 8.1 and previous versions.

How are you trying to open the saved MHT file?

File > Open File... or dragging the saved MHT file into the Firefox window works for me with UnMHT 5.7.0 in Firefox 10.0

Dragging the MHT into Firefox, and File > Open both work, but I prefer to be able to click the file and have Firefox open it.

I also tried right-clicking the MHT file and changing "Opens with" to Firefox, but no luck - Firefox does not appear on the "Open with" list.

I wonder if this is an XP problem.

It's coming back to me how I fixed this problem when it happened before. I deleted all references to Firefox in the Registry - very tedious, hope I don't have to do it again.

I found this: http://forums.cnet.com/7723-6142_102-24218/file-association-problems/ but am wary about running something that might make things even worse.

That probably isn't going to help you, Firefox Portable doesn't write to the Registry.

Then maybe that was somehow related to the problem the first time it happened because there were many many instances of Firefox.exe.

In any case, after messing with it most of the day, downloading and installing Firefox 10 portable after deleting the old versions (there were a few) after saving the profile (forgot about the bookmarks and lost them) it now recognizes MHT files and opens them properly.

Thanks for your help ... I won't be upgrading Firefox again for a very long time.