Firefox is, in my personal & professional opinion, a superior product to that of Microsoft's Internet Exploder.
Where there is one thing that Firefox won't do correctly, there are ten things that Internet Exploder won't do correctly.
I once was an IE junkie. Didn't think I needed anything better. Up until an update of IE Version 6, Microsoft peeled itself away as the W3C's leading compatibility partner into a ringer with Nutscrape. Bad move Microsoft. IE7 was a feeble attempt at keeping up with what other popular browsers were doing.
Firefox now owns a healthy 37% of the browser market share (see citation
IE5, 6 and 7 combined still lead the way with just a smidge over 53% market share -- but we must remember, that also accounts for all those old grannies and grandpas who have a shiny Windows XP or Windows Vista computer and would never even think to 'upgrade' their web browser, or consider even what a web browser actually is.
Also consider this minor pain in the ass -- Internet Explorer 6 is still hanging on to 30.1% of the market share. This is frustrating as a web developer because of IE6's poor PNG-24 support in which the major problem is a lack of alpha transparency -- a technology that many of today's graphics are being built on.
Never the less all browser competition aside, when you have new messages, the flashing flag is shown. After you've gone in to read your messages, but perhaps have left one 'till later, the 'space' John referred to is there, but without the flashing 'new message' indicator. I don't believe it's an error as much as a misconfiguration of sorts in which perhaps an image is missing or something.
At least thats my best educated guess.