This is because we all had to "hack" 10.04 to get anything above Firefox
3x installed in it. Odds are that Ubuntu doesn't recognize that 10.04
could be running FF4 or FF5. The Ubufox update being sent out by Ubuntu
for 10.04 is most likely for FF3x. I'll bet that there is no Ubufox
update from Ubuntu for FF4 or FF5 for Ubuntu 10.04.

This is what my update manager says:

Changes for the versions:
0.9-0ubuntu1~mfs~lucid1
0.9.1-0ubuntu0.10.04.1~mfn3

This change is not coming from a source that supports changelogs.