Works with some flaws in FF16 Rated 5 out of 5 stars
I updated the add-on to work with FF16 by downloading it first, using right click > save as and then editing the maxVersion as described in http://kb.mozillazine.org/Editing_an_add-on_to_change_its_compatibility Increase the value for maxVersion to f.i. 19 as others have already written. I have found that I have to install the adapted add on after updating FF to version 16. When I did it before the FF update, I still got the incompatibility error. The add-on can be installed by opening it in FF or simply dragging the xpi file to FF.
If you have lost the option Tabs on bottom, you can make that option available again by following the instructions on https://support.mozilla.org/en-US/kb/put-tabs-back-on-bottom.
So far I have found that the add on does not work perfectly anymore in FF16. If I click f.i. the green area in the address bar if there is a certificate for an HTTPS connection, the text balloon is now transparent instead of solid. The same happens when clicking the bookmark star in the address bar. This is not a big issue for me, so I’ll keep using the FF3 theme in FF16.
I have given this add on a 5 star rating before and have already given a donation to the maker, Jeremy Morton to thank him for his good work. If the add on is maintained again, Jeremy, I promise I will give you another donation. Hopefully Mozilla decides to keep “Tabs on bottom” alive. If Mozilla does decide to eliminate it though, I hope the "Tabs on bottom" add on (which is in the making by Brandon Cheng, as I read on https://bugzilla.mozilla.org/show_bug.cgi?id=755593) will be sufficient to keep the FF3 theme alive.
For now, I urge everybody who is dissatisfied with the possible demise of Tabs on bottom and therefore this add on to post a comment on https://groups.google.com/forum/?fromgroups#!forum/mozilla.dev.apps.firefox.