tordev123@Safe-mail.net:

Is there some documentation, how the tor-browser git repository is set up?

Not sure what you mean but it is a repository that contains Firefox ESR code and on top of that our patches we need to apply. The branch names encode the ESR version and the Tor Browser version: e.g. the branch for ESR 38.2.0 used in Tor Browser 5.0 is tor-browser-38.2.0esr-5.0-1.

In particular, how are new Firefox releases imported? How can I get a diff of Tor changes to the underlying Firefox release?

We are using the git mirror at https://github.com/mozilla/gecko-dev, the esr38 branch in particular. Importing the Firefox releases is still done by hand as the canonical repository delpoyed by Mozilla is using mercurial and finding the right commits is not that trivial sometimes. Looking at https://hg.mozilla.org/releases/mozilla-esr38/graph/ should give you the branch you are looking for. The content of the last commit there is the content of the last Mozilla commit in our respective branch. To find that mapping you could use https://wiki.mozilla.org/ReleaseEngineering/Applications/Mapper#GET_Routes or whatever fits your need. All commits on top of that commit are our patches. Georg