As discussed in Github, the Nightly Tester Tools Extension needs work to become a webextension. Henrik doesn’t have the bandwidth to continue work on the conversion, so the extension will be end of life with FF 56 unless someone wants to help. Two questions:
Is anyone in the community interested in helping to finish the work Henrik started?
Henrik and David would be interested in how many people are actually using the extension in their daily workflow.
A necessary question, which features are necessary and achievable? And, which API proposals are required.
I’ve been trying to get more involved in the Nightly build but alas I can’t get the build running properly, It doesn’t detect that I have got a 64 bit OS when I run the config and build. Since the build docs aren’t going to be updated anytime soon it has made it difficult to contribute to the project.
So my questions are what exactly is the Nightly Tester Tools and do you know if they will ever have an ETA for the build docs to be updated?
If and when I get around to fixing the build I would love to be able to help out with things such as this.
( I have wanted to debug the build myself and get help from the IRC channel but have found it hard to find the time to go through and do in a timely manner myself )
~ Regards, Steve
Keep Building 
I think @kohei.yoshino already started some work on a WebExtension version of Nightly Tester Tools. If necessary, I may help with future development/maintenance, but not before September, as I am quite busy until the end of summer holidays. 
To me the most used feature is to get my build info to insert into bug reports.
Should have updated this thread earlier. I just sent a pull request to merge my WebExtension port, will figure out how to implement more features found in the legacy version 