@Thane: Thanks so much. This pretty much disconfirms my working hypothesis, at least for Thunderbird -- that going into Expo view is necessary to trigger the problem -- but disconfirmation is useful information nonetheless.
This seems to be a nasty bug in that I suspect it's going to be a hard one for the developers to nail down. It's specific to Mozilla software, and seems to be a bad Mozilla / Unity 3D / Compiz interaction, possibly also specific to certain video drivers (as not everyone has this problem -- 68 affected users is a lot, but I suspect small compared to the active Ubuntu user base who would take the time to report it).
So, the more we can do to narrow down the boundary conditions -- or define some usage case that _always_ causes the problem for all users affected -- the more we can help the developers isolate the problem (obviously). Unfortunately, I'm not sure what to suggest next, although one question does come to mind:
Is anyone having this problem with a _fresh_ install of Oneiric (11.10)? (I for one upgraded from a 11.04 fresh install to 11.10 on its release date.)