Well, not fixed, but I have a hack/workaround that will do for now. Apparently this pixel shift was deliberately put in to Wordpress to accomodate a bug in IE, and Firefox 1.0.x ignored it, but Firefox 1.5 RC3 follows the CSS standard more closely and doesn’t ignore it. There is no ‘maybe’ about it – Firefox is following the W3C CSS specification therefore Firefox is in the right and Wordpress is in the wrong, but for now I have a solution available that presents the IE-bug spreadsheet to IE users, and presents the normal stylesheet to non-IE users and people with javascript disabled (since javascript is the detection method). The detection method is not foolproof (*shudders*) – any Firefox users with the ActiveX plugin (can’t for the life of me think why since ActiveX is one of IE’s biggest flaws) may experience the pixel shift (since the nifty browser detection works by checking for ActiveX, which normally only IE has). Sorry folks, but it’s the best I can do!
UPDATE: The browser detection didn’t work (I forgot that XP SP2 tightens up ActiveX which presumably includes preventing javascripts from detecting it :P) so it’s stuck the way it is for now. If you have a problem with this (you should :P), complain on the Wordpress support discussion.