Matt - Thanks. That worked for me. It's a work around that will
suffice for the moment.
Specifically (to spell out Matt's directions a little more):
* I brought up Ubiquity via hot key.
* I typed "twit ...some status update..." and hit enter
* At this point ubiquity went away and the menubar became just
"Firefox" next to the Apple logo in the upper left.
* Without doing anything else, hit the "Return/Enter" key again on the
keyboard.
* The menubar returned to the normal full Firefox menubar and I got
the growl notification that my twitter status had been updated.
* Subsequent updates seem to work just fine.
So for some reason the twitter login window isn't being displayed, but
it has focus. The way it appears/disappears in expose makes me thing
it's being rendered "off-screen" somewhere, i.e. I see it fly from the
upper left and then back out that way.