I've been using only this for the last 10 years or so (on windows, Linux, Mac and Android), and I can count the number of time things really broke on one hand.
We call these Firefox Nighly, and they are available at https://www.mozilla.org/en-US/firefox/nightly/all/.
Release notes at available at https://www.mozilla.org/en-US/firefox/71.0a1/releasenotes/, but they are not always updated for each version, sometimes batched (its quite some work due to the amount of commits that are pushed per day).
There is a cool blog at https://blog.nightly.mozilla.org/ that talks a bit about the new stuff.
All this is really experimental and probably not for everybody, but helps us to deliver something good at the end. Filing bugs and giving us feedback is of course particularly appreciated, as always!
*impossible with a limited constraint of time and/or money.