You can use the LTS (Extended Support Release? Not sure about the Mozilla terminology) version. For a while..
It's called Firefox ESR
The next Firefox ESR will be based on Firefox 52 (to be released in March). Firefox 52 ESR will continue to support all add-ons and receive security fixes until sometime in 2018 H1.
Is there a Patreon for that?
Pale Moon
Thanks for the tip on this - installing.
However, that’s pretty much the other extreme. It doesn’t support Web Extensions or even Add-on SDK extensions.
Do you use any indispensable add-ons that are Web Extension based?
No. However, I am the developer of "Download Cleaner", which is made using Add-on SDK. I think it's a great solution to get started without learning XUL.
I'm sure they had their reasons for deciding what they did. I still think it's a bad decision.