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I was looking at their site and it does look like their support information is a bit out of date (mentioning a 7x Opera release).
Uhm.. I think their project is starving...
Anyway, FF is the most standard-compliant browser, and I do believe that other browsers can only improve their compatibility with new versions.
Do you think it would be possible to create a further "windowed" manager layout? (don't forget that the current "drop-down menu" layout works only in IE). It doesn't seem difficult to obtain, and it would be frameless. Just imagine: the Document Tree in a window, the Menu in another element (like the Windows "Start" menu); and every time an user selects a menu command, a new window pops up with the selected action. In this way, it could be possible to edit many pages at the same time, to easily copy-and-paste text between pages etc. Furthermore, the windowing system supports message passing, so it would be possible, for instance, to trigger an update of the tree when a page is saved.