( FIXED )
(Ed. note - several users have asked about this problem in Ask Email Guy, so I posted the answer here rather than publish the individual questions)
After the release of Web Mail 5.13 on Monday 3/5, a few users reported that the subject lines in Web Mail would disappear or not be clickable. As far as we have been able to determine, this problem is confined to old versions of Mozilla browsers and old versions of Firefox browsers. In every case the issue is fixed by updating to a newer version.
Update Firefox here:
The problem never occurs on Firefox 2.0. We have a few rare reports of it on Firefox 1.5 if the user has a lot of messages in the Inbox. But if you still have a really old 1.0.x version of Firefox or Mozilla, we can no longer support it, and unfortunately you'll need to get a newer version. We do still fix bugs related to Firefox 1.5, and we will continue to test on it for a while. But we recommend that you update to version 2.0 at the web site above.