I just tested Seller Details with Firefox 3.0.5 (latest) and IE7 on WinXP Pro with Service Pack 3. No problems here.

Just installing a new computer (last round of testing), so everything on my end is fresh installs.
Danno
Edit: Given the tip by egsherish that Firefox 3.0.6 was released (since my very recent install), I updated from 3.0.5. I found no change in working status, Seller Details works as usual.
The closest I can come to replicating your problem is to disable Javascript in Firefox (see Options - Content menu). That toggle between Standard and HTML mode works with an anchor and onClick script. Check your settings. Also, an add-on or security product might interfere with the functioning of the script.