Rated 5 out of 5 stars

As posted, hacking the .xpi file works like a charm! Thank you very much! Instructions copied here, AGAIN, to try to help others to get Duplicate Tab going on V3.5.2.

From mikedepetris ( http://forums.mozillazine.org/viewtopic.php?f=48&t=83785&start=165 ):
- download duplicatetab-1.0.2.xpi
- rename it to duplicatetab-1.0.2.xpi.zip or .rar
- extract install.rdf with a suitable archiver
- edit install.rdf
- change the line:
3.1a1pre
to
9.9
(this makes it good up to FireFox 9.9)
- save install.rdf and repack it in duplicatetab-1.0.2.xpi.zip
- rename it to duplicatetab-1.0.2whatever.xpi
- open the .xpi file and install the extension
- play :)