Hi there!
Thanks for reporting this problem.
We fixed this about 4 months ago, but the folks at Mozilla have been
very slow at updating their web site. (We pushed the latest update to
them about 2 weeks ago)
If you want to grab the newest version directly from MIT you can go
here:
http://welist.it
The newest version is 0.4.9.9 and is tested and works with Firefox 4!
Thanks!
max
list-it team lead