This isn't a dup of 100921 then?
Looks like a dup of bug 100923. (Bug 100921 is about components not receiving focus, although the cause for both was the same.) I cannot reproduce this with Build 2001092521 on Linux.
build: 2001092503 Windows 98 I followed the directions above, and the focus went from the text field to the first button (OK) as expected. I pressed Tab again, the focus went to the second button (Cancel) as expected. Tabbed again, and focus went back to the first button. I expected it to go to the text field instead. Pressed Shift-Tab (when focus is on the first button), focus went into text field as expected. Shift-Tab in the text field causes the symptoms given in the above description. I killed and restarted Mozilla, created the popup, and pressed Shift-Tab right away. Same results -> hang.
I think this was fallout from 100921, which bryner checked in a fix for on 9/25. In current builds, on either Mac or win2k, I can't reproduce this hang as described (even in Classic on Mac, where the buttons aren't focussable). Reopen as required...
Verified worksforme, build 2001092808, Mac OS 9.1.