That page is very out of date. They did later adopt a build step and switch to using JSX in much of the FF DevTools codebase.
As an example, here's the Editor main component (same "latest" commit you pointed to):
https://github.com/mozilla/gecko-dev/blob/aec3a901e6f6b3041b...
The codebase is a mish-mash of very old-style React+Redux code, with newer files and usages.
(I know this because I work at Replay.io, which started as a fork of the FF DevTools source, and we spent all of last year modernizing our fork :) Migrated the entire codebase to TS, ripped out all remaining uses of the ancient `React.DOM` helper functions and converted those to JSX, modernized the 2015-era Redux logic, etc: https://github.com/replayio/devtools/pulls?q=is%3Apr+sort%3A... )
