Firefox 3.0.1 on Mac OS X weirdness

So I ran into an interesting problem today. A site I built (validates as XHTML
1.0 Strict and CSS level 2.1 and runs in Standards-mode instead of Quirks-mode
in Internet Explorer 6.0) renders great in Firefox 3.0.3 (tested on both on
Linux and Windows), Internet Explorer 6.x (tested in Wine and Windows) and
Safari (tested in Mac OS X and Windows). Unfortunately, it wasn’t rendering
properly (a side bar was displaying way too wide and was breaking the layout) on
the client’s installation of Firefox (version 3.0.1). My resident Apple fanboy /
guru (his term not mine) tried it and it seemed fine on his Mac running Firefox
3.0.3. Well I restarted my Firefox today for the first time in ages (yeah I’m
that guy) and noticed that there was an update to Firefox to be installed,
3.0.3. Wait, the client had 3.0.1 didn’t they? “Hey client, can you check if
there’s an update for Firefox on your computer, just for shits and giggles and
all?” “Hey, there was one, weird because I have automatic updates on as well…
oh and look at that, it’s working now” “Are you shitting me?” “No, seriously it
looks fine now, must have been a bug in that version.” Must have been, probably
will never know. Anyone else ran into any weirdness like that with Firefox
3.0.1?