I was hoping that NTT's title-bar editor would do what I want, but it doesn't, so probably the only feature I'll really use (not actually being a nightly tester, just beta) is the "force compatibility" tool, in order not to keep losing add-ons every time I update to a new Beta release number. The other tools do look useful for people who really are living on the Firefox edge.
What I was *hoping* the title-bar editor might do was allow me to customize the title *individually* for each of my open windows. I've lately been trying to use separate windows to group related tabs (e.g. a "social" window for Facebook and LinkedIn tabs, a "work" window for things I'm currently researching for work, etc.). I often have several hundred tabs open (lots of "I'll get back to that"s that never happen...) but it's hard to keep track of what's where when each window just shows the title of the current web page. And no, Firefox's own "tab groups" feature won't do what I want, and I hate, hate, hate that interface. (Clustering *actual* tabs or expand/collapse *actual* tabs would be the way to go -- not a gazillion thumbnails.)
UPDATE -- 25 Feb 2013: Loss of 1 star, from 4 to 3.
I just had my first occasion to try using the "Force Addon Compatibility" feature, when FF updated itself from Version 19 to Version 20 Beta 1. (Does anyone else find it absurd that we're up to "FF 20" with essentially the same product as FF 7? Or is it FF 5?) As a result of the update, my "Tab Utillities" add-on was disabled as being "incompatible".
I saw a warning about the Tab Utilities add-on before I actually applied the update, so I turned on "Force Compatibility" before updating FF. No joy -- the add-on was still disabled after the update. So I turned off the Force Compatibility, then turned it back on and accepted a re-start. Still no joy, the add-on continues to show as disabled. So, as far as I can tell, the Force Addon Compatibility feature doesn't work. Since that's the only feature I was expecting to use, I've deducted one star.
What does work (sort of), in place of NTT, is the trick COKEDUDE posted below, updated. I added
to about:config. Now I can fiddle with Tab Utilities options, but it doesn't seem like Tab Utilities actually WORKS even with that new Config line.
(As a feature request for more serious testers than me, I wonder if forced compatibility could be managed on a per-add-on basis. Someone doing serious testing might want to activate just one "incompatible" add-on at a time to see if they actually work.)