Me too.
I'm using i386 FC4 (up to date as of 2006/01/21), and initially installed binary 3.0M3_436 and then upgraded to 3.0M4_41. The install and upgrade went without incident. In addition to the 'thin bar' problem (Firefox), my 'reading pane' is always blank, regardless of 'conversation' or 'message' grouping and browser.
I've tested this on:
WinXP: IE6, Firefox 1.5
Mac OS 10.4.4: Firefox 1.5.0.1, Safari 2.0.3 (417.8)
Linux: Firefox 1.5
and every one of them is the same. This so such an obvious probelm, I must be missing something in the configurations. So, for the users, I restored the defaults, but the problem persists.