swfupload hanging in FF 9&10

After a few uploads in a multiple upload on an imagefield (In FF9,10) the swfupload widget freezes.
In windows 7, it happens after 10-15 files have uploaded, and on Mac 10.6&10.7 it happens between the 1st and 4th file.
Tested with Flash versions 9-11 with the same results (on Mac 10.6.8 only).
Intermittently, I am able to see an uncaught exception on the function SetStats in firebug.

I typo'd in the title of this issue. This problem does not occur in Firefox 8.