I suspect the latest upgrade to WordPress (I'm assuming they went from 2.3.1 to 2.3.2 on the multi-user version as well on used here) seems to have caused some problems. Firefox for me is still somewhat hit and miss. I opened a previous post tonight to add some tags and Firefox stripped out all my paragraph breaks, but while playing around and testing earlier today with Firefox everything was working. Since I haven't been able to get Firefox to consistently "mess up" I haven't really been able to test any workarounds.
I wish I had better news or a solution, but I do not. One thing you can do if you wish, it to try using safari after turning on the debug menu and then setting the user agent to "Firefox". That has been working well in my testing with no issues I've found yet.
It requires a trip to the terminal program in applications > utilities. First quit Safari. Then with in the terminal window type in:
defaults write com.apple.Safari IncludeDebugMenu 1 and then hit return.
The open Safari and in the "Debug" menu, set user agent to "Firefox 220.127.116.11. You should then be able to write posts just fine in Safari complete with paragraph breaks.