Thanks to a posting by “Bountyhunter” at http://forums.macosxhints.com/archive/index.php/t-63868.html it’s now fixed. To quote the post:

I came across this post while I was trying to fix the same problem, and I found the solution is that the download history is extremely full, and takes a long time to load, thereby hanging up Firefox for a while. Click the “Clean Up” button at the bottom of the “Downloads” window, and it’ll be fixed. You can also set it to do that automatically for you under Tools->Options->Privacy->Download History

http://billstclair.com/blog/slow_firefox_downloads_fixed.html

and

http://kb.mozillazine.org/Firefox_hangs#Hang_downloading_files