We're actually getting some interesting data coming it from the instrumentation "startup ping".
In particular, you can see that people updated from Firefox 1.5 to 1.5.0.1 almost the moment it was released.
We can also estimate that there are approximately 1100 new or updated ChatZilla users a week (calculation: sum up new installs [the numbers on the pages linked, 40], divide by the number of days [25], multiply by 700 [1% and we want per-week]).