We tested the 1.9 release on 2 different Macs and it worked fine so I guess there must be something specific to your computer. Do you have any other add-ons installed alongside? If so, try disabling one by one and see if it works. You could also try with a new fresh Firefox profile (profiles can cause a lot of stuff not work on FF4+. Then you could try to re-install Firefox.
I will try to find another user with OS X 10.6 and check with him but honestly, the add-on uses only official Firefox API so it shouldn't depend on anything else. Did you try clearing cache with the regular dialog (Tools - Options - Advanced - Network - Clear now)? Does that work?
If you're not sure how to check if cache is actually cleared, open 'about:cache', clear cache and then refresh 'about:cache'.
EDIT: I got one volunteer to test 1.9 on OS X 10.6 and it runs great.