Because of bug 703031. We have the same problem with Tilt (bug 719039). We will probably use the same method that Tilt will use to fix this bug.
No need to track - please nominate for Beta 11 approval if you'd like to make the case for this being uplifted.
Verified as fixed on: Mozilla/5.0 (Windows NT 6.1; rv:12.0) Gecko/20100101 Firefox/12.0 Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20100101 Firefox/12.0 Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:12.0) Gecko/20100101 Firefox/12.0 20120328051619