It will block unsigned extensions by default in 43, but you can disable it: set xpinstall.signatures.required to false in about:config. This option will be removed in 44. (https://wiki.mozilla.org/Add-ons/Extension_Signing#Timeline)
"The Developer Edition and Nightly versions of Firefox will have a setting to disable signature enforcement. There will also be special unbranded versions of Release and Beta that will have this setting, so that add-on developers can work on their add-ons without having to sign every build."
For my main browser that has all the plugins I used to appreciate in Firefox I switched to Pale Moon [1]. On other machines I continue to use Firefox.
Very happy with Pale Moon, works perfect -including all my old plugins- and I'm no longer afraid that features I depend on will stop working because the mozilla dev's decided to try and change firefox into chrome.
If you don't care for tab groups - my experiences with Pale Moon were great! I can imagine it's remained as awesome since I stopped using it (which is when they removed tab groups...)
E:
Also it's totally possible the add-on has been fixed. But I don't remember anyone maintaining it, it was just segmented away from the browser and made into an add-on without development/maintenance. It's possible that has changed and the above issue is no longer relevant.
You are correct that tab groups is offered as an add-on. Had no problem installing it, it even kept all the tabs I had in tab groups in Firefox. For me the transition from Firefox to Pale Moon was very smooth.
[1] http://www.ghacks.net/2015/11/08/mozilla-to-remove-tab-group...
If those two issues have been fixed I'd likely very quickly switch back to Pale Moon.
As an aside, I'm aware FF is removing them as well in the near future. I've already frozen myself on FF 40 for other reasons. Before anyone mentions - I'm aware of the potential security risks of doing so and have decided given my internet browsing habits it is very low risk. Low enough that I don't even consider publicly announcing my use of FF40 to be a significant security threat.
But -if it helps- the tab groups are remembered after I kill firefox from Windows task manager.
Also no problem on updating Pale Moon. But I've got to admit that there was only one update since I switched over to Pale Moon.
https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta...
I'm currently running Firefox 43 on Arch Linux:
$ pacman -Q firefox
firefox 43.0-1
That extension has been clearly signed.