FOLLOW UP: It appears the latest Firefox update 3.5.7 may have resolved this issue.Problem: This is a very interesting problem that I had and it only existed in Firefox 3. I use list to organize friends from work, home town, etc. I recently added a friend to one of the list. When I went back to All Friends and selected the friend list that I added them to, they did not show up.
While checking the problem I was curious is IE 8 and Google Chome had the same problem and they did NOT. So now I had it isolated to a FireFox problem. I spent about an hour trying to figure out why it was not displaying. I out of curiousity figured it might be a caching problem so I tried deleting offline files and reducing the size to 1 MB. This made no difference and the problem still existed.
Out of frustration I uninstalled firefox and reinstalled it, which to my surprise the re-installing Firefox did NOT fix the problem.
I will post a resolution if I find it, but I just wanted to get this out there so you would know your are not crazy in case you have the same problem.
FOLLOW UP: This is very interesting. I didn't find a fix for firefox, but I did determine it is definetly a Firefox unique problem. IE 8 and Google Chrome to NOT have this problem. I have or at least tried submitting a trouble ticket to Firefox, but it was a pain. Seems they don't take problem reports very easily.

Anyway if you have this problem, use another browser to edit your lists.