With the release of Firefox 57 towards the end of this year, Mozilla's browser will no longer support "legacy" addons. The switch to WebExtension means developers will have to almost certainly update their addons to make them compatible. This shouldn't be an issue for popular addons, but if you're using lesser known or outdated ones, you'll need to take steps to make sure you don't get caught out.
As gHacks' Martin Brinkmann suggests, you can just go with the Extended Support Release of Firefox, which currently sits at version 52. While the ESR will receive security updates, there's little to no chance of getting backported features, optimisations and other fixes.
Alternatively, there's Waterfox. A Firefox fork of sorts, developed by Alex Kontos, it was the first attempt to build the browser as a 64-bit client.
Kontos has stated that "addon support will carry on" in Waterfox for "as long as feasible", via the browser's own ESR. It's hard to say how much this will differ from Mozilla's, but there's a better chance of getting future fixes and improvements integrated.
In addition, Kontos has his own plans to keep Firefox's legacy addon system alive going forward:
In parallel to that, I was planning to go and try and get funding to create a startup using Waterfox's name to keep XUL/XPCOM alive and modernise it, essentially forking Firefox from its last usable XUL/XPCOM version and creating a new browser based on all that. Get some hard core software engineers/programmers on board and try and follow in Mozilla's ethos that has sort of been my guide as a programmer.
It'll be interesting to see how long the "if it ain't broke..." philosophy works out for Waterfox in the long run.