Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Firefox 3.6 will be the smallest upgrade Mozilla has released to date. Before this, version jumps have been in the 0.5 (Firefox 1.0 to 1.5, 3.0 to 3.5) or 1.0 (2.0 to 3.0) order, and while version numbers are not mathematically representative, the slight 0.1 version increase reflects the overall tone of this update, and Firefox development in general which is moving to a faster release pace.
Without a doubt, the most significant addition is support for lightweight themes, also known as personas, that was previously only available with the Personas extension.
A persona, which consists of a couple of images used as backgrounds for the toolbar and status bar areas, and setting a text color, is very simple to produce, yet it significantly changes the appearance of Firefox and easily becomes a way of expression for the user.
To try it, simply visit Mozilla Add-ons, and go to the recently added Personas section. Personas are live previewed as you hover on the thumbnails, and become permanent when you click on Wear It.
Themes after all, personas are also managed via the Themes page in the Add-ons Manager.