Testing against Nightly, Developer Edition, ESR — or a fork

Every command that opens a browser takes a channel name as well as a path:

fxcss watch --firefox nightly
fxcss audit --firefox dev          # what will break before it ships
fxcss shot  --firefox esr --out shots/esr

Recognised names: stable, beta, dev, nightly, esr, and the Gecko forks theme users actually run — librewolf, floorp, waterfox, zen. They resolve against what is installed in the usual places; a build kept somewhere unusual can be added with FXCSS_FIREFOX_ROOTS=/path/to/dir.

With several builds installed and no --firefox given, interactive commands show a picker — press Enter for stable, or a number for another build. CI and scripts are never prompted: non-interactive runs keep the old behaviour exactly.

These names select browsers already installed on your machine. The local commands do not install or update Firefox.