The menu bar is borked:

Menu bar and tool bar are blacked out

Menu bar and tools bar are blacked out

I have to click inside the bar to get a drop-down to appear

Clicking inside the bar gets a visible drop-down menu to appear

But no worries, IE Tab in Firefox still works just fine :)

Update 1/29: Upgraded to RC1 yesterday (forcefully, I might add, as I thought I had removed the beta and reverted back to IE7, which didn’t fix the problem.) The same results as above when I launched the browser, but it seems to have disappeared when I fired it up today:

ie8rc1