Works well Rated 4 out of 5 stars
Looks like now I might be able to avoid those ugly Australis tabs entirely. :D Your planned color etc. options can all be done now with CSS--so simply providing that will help some users [who keep complaining about the bg color :D] right away (while you work on the UI). I think you really need to highlight the New Tab "button" in some way, esp. on hover.
The tab drop indicator doesn't work properly--you start dragging and it appears up on the Bookmarks toolbar and doesn't move (so, not "indicating" at all). I don't drag tabs much anyway.
(FWIW: the right side position option doesn't seem to work in Fx16beta--working OK now for the release update (16.0.1); don't know if that means you'll have to change something soon or if there's a bug there that they haven't fixed yet.)
Anyone who wants to customize their own installed version--colors and what not, feel free to use any code you find here:
UPDATE: Well, now that I've had a chance to try out all(?) of the several add-ons for "vertical tabs", I mostly prefer this one. It's the closest one to being what I want: simply making the horizontal tab bar vertical. And I say this in spite of its one major "sin": positioning itself on the right by reversing the normal browser, which takes EVERYTHING on the left and puts them on the right (big no-no). Anyway, all of them have some problem or other. This one's are just easier to work with [for now--I guess I'll check out the others again in a few months to see if they've "improved"]. So, whatever you do regarding your planned updates, please remember to just... Keep It Simple S*****. :D Thanks.