A few months ago they released a version of Nightly that would crash for most users on startup every time. It was up on the Play store for several days.
That would normally be a buyer-beware situation as with any dev build, but Mozilla had purposely locked several important features - such as about:config configuration and loading any extension - behind the Nightly version, forcing users into that kind of experience.
Most of the problems I have are that when opening a new tab, FF does not show the content. I need to copy paste the url to another tab, and then it works.
I have never had mobile FF crash on me, largely been a good experience.