Grondar, the place for Mozilla add-ons is
http://www.mozdev.org - great site.
themes.mozdev.org <--- themes
The only other site one would ever need for Mozilla:
www.deskmod.com <--- more themes, most of which are also on mozdev.
As far as all links opening in tabs, look around in Mozilla preferences. You can set it so that middle clicking a link opens it in a tab and just tdo that from then on
If you REALLY must have it be on the left-click, perhaps there's an add-on (I doubt it) but since it's open source, you can just learn to program and add it yourself!
(I know this isn't really a possibility to most people, but the fact it's there is part of what makes Mozilla great. It CAN do anything you want it to.)