Great Persona Manager Rated 5 out of 5 stars
This is a nice way to manage personas (lightweight themes that only change the toolbar graphic, leaving the icons alone). You can hover over each theme, and see what it looks like without actually changing the theme. In the FF default theme manager, you have to enable the theme, and if you have a lot, you might forget which one you had originally. Helpful.
One bit of advice--I found that the Compact View was easiest for me to use--all I want is the ability to enable/disable each theme; the default view gives you all sorts of incidental crud that the theme author inserted.