This is great, thanks for posting it. I manage my passwords with 1Password, and while they're good about pushing updates, I've never seen their Firefox extension marked as compatible when a new version of Firefox is released.
1Password 3.8.7 seems to be compatible with Firefox 8 though. They're getting a lot better at keeping up with Firefox version lately (ever since they introduced Aurora and "maybe" Nightly support)
Why 1Password rather than Firefox Sync?
Probably because 1Password doesn't support just a specific browser and it's also available for tablets and smartphones.
The Sync API is documented, and Mozilla encourages development of outside server and client implementations. Sync is available for tablets and smartphones.