I think I found the problem, but I don't know how to fix it. This is how the avatar location is displaying when I view the source:
<img src="http://www.matesupover.com\forums\images\user_avatars/35.gif" alt="" width="100" height="100" />
See that the slashes are going the wrong way. Avatars that were imported with the old database show the correct slashes.
I looked at the IE source, and the slashes are going the wrong way too, but IE doesn't seem to care, while Firefox does.
Again .. remember that this is only happening to avatars that are changed since changing to 7.1.
Any suggestions on how to fix this?