Possible replacements for Tab Mix Plus: Obviously there's no drop-in replacement for everything. Even with that aside, it's hard to make recommendations because it had so many features and probably a few were especially important to you and they might differ from the ones that were important to me.
For me, the number one feature originally was to force absolutely all popups to appear in tabs, not windows. This is actually just a built-in option now (maybe it was all along?) [1]
I used to use a feature where you could hold down the right mouse button and use the scroll wheel to switch tabs - I can't remember now whether that was provided by Tab Mix Plus or FireGestures. Anyway, now it's in FireGestures' successor, Foxy Gestures [2].
But the main recommendation is Tree Style Tab [3] (plus its own extensions [4] e.g. I use TST mouse wheel, and there seems to be a tab colouring one if that's your cup of tea). The key thing is that TST draws the tabs itself so it can make any manipulations it likes to them, which it can't do with the original tab bar in new extensions (you can hide the original tab bar [5] to avoid confusing and wasteful duplication).
I realise that laying your tabs out in a totally different way is quite a big change but it's actually an amazing change by itself. It's a much more logical way to layout tabs, and you can see more tabs at once (without resorting to multiple rows - yuck!). Once you get used to it, using a linear tab bar seems ridiculous in comparison. Plus extra vertical screen space is useful and horizontal space is typically just wasted. I recommend playing with its options a bit though; I find its default behaviour (collapsing trees automatically when you switch to another one) a bit frustrating.
For me, the number one feature originally was to force absolutely all popups to appear in tabs, not windows. This is actually just a built-in option now (maybe it was all along?) [1]
I used to use a feature where you could hold down the right mouse button and use the scroll wheel to switch tabs - I can't remember now whether that was provided by Tab Mix Plus or FireGestures. Anyway, now it's in FireGestures' successor, Foxy Gestures [2].
But the main recommendation is Tree Style Tab [3] (plus its own extensions [4] e.g. I use TST mouse wheel, and there seems to be a tab colouring one if that's your cup of tea). The key thing is that TST draws the tabs itself so it can make any manipulations it likes to them, which it can't do with the original tab bar in new extensions (you can hide the original tab bar [5] to avoid confusing and wasteful duplication).
I realise that laying your tabs out in a totally different way is quite a big change but it's actually an amazing change by itself. It's a much more logical way to layout tabs, and you can see more tabs at once (without resorting to multiple rows - yuck!). Once you get used to it, using a linear tab bar seems ridiculous in comparison. Plus extra vertical screen space is useful and horizontal space is typically just wasted. I recommend playing with its options a bit though; I find its default behaviour (collapsing trees automatically when you switch to another one) a bit frustrating.
[1] http://kb.mozillazine.org/Browser.link.open_newwindow.restri... (set to 0)
[2] https://addons.mozilla.org/en-GB/firefox/addon/foxy-gestures...
[3] https://addons.mozilla.org/en-GB/firefox/addon/tree-style-ta...
[4] https://github.com/piroor/treestyletab/blob/master/README.md...
[5] https://github.com/piroor/treestyletab/wiki/Code-snippets-fo...