I'm using Mozilla Firefox .8 on Windows, although I think the same thing happens on Firebird .7+.
When I browse tiki.org without being logged in, menus have a blue background. When I do the same on IE, the menus have a white background.
I have "moreneat" as my style. When I log into tw.o using Mozilla, the menus change to a white background.
I'm using Firebird since monthes, 0.7 never did that under Windows or Linux. I'm still waiting a RPM for Firefox, (yes!) and I'll tell you what I get when I install it.
Something that could be related : IE has application menu partly broken for 1.8 (including tw.o).
More followup : as mose said on IRC, this is just mozilla.css theme. I don't know why you see it at that time though.