If I:
1. Clear cookies in both browsers and close both.
2. Open Firefox, visit the forum and log in with the "automatically" box checked, close Firefox.
3. Open IE, visit the forum and log in with the "automatically" box checked.
4. Open Firefox and visit the forum.
I am still logged in in Firefox. What happens if you run through that sequence?
Perhaps there needs to be a delay, so I'm going to alternate between Firefox and IE over the next few days to see if I run into the problem. I assume it's only started occuring recently, and it may have more to do with phpBB than Firefox 1.5. It's mentioned in
this thread that there was a change in the way auto-login cookies are set in 2.0.18.