I'm having the exact same problem, only Firefox is not listed in my login items, and Firefox is not set to Open at login. Is there any way to resolve this?
Editing this post for future reference, in case anyone else experiences the same issue: After a ton of frustration, I finally figured this out. However, after uninstalling Firefox, it still kept launching during login. This is not possible, right? Wrong! I had Firefox bundled in an old installation of the TOR Browser/Vidalia. Uninstalling the bundle resolved everything. I have never been more flabbergasted as I was after seeing Firefox still starting up after uninstalling it, not realizing I had another installation hidden in the aforementioned bundle.