We could make local development and Try really fast if we didn't require (re)compiling everything for changes to non-compiled files. I'm imagining something like # mach build --use-binaries https://.../firefox-nightly-win32.exe This would download/unpack the specified binaries, and swap in local non-compiled files, and then optionally repack the result. We could then add try syntax support for this, or allow in-tree manifests to specify binaries to use. Or perhaps the default case could even be to re-use existing nightly binaries if we can determine that all you've changed are .js files in your try patch.

For the record, I do something very similar to this with Android Gradle builds. (This hasn't entirely landed yet.) ./mach configure && make -C objdir export misc chrome && ./mach gradle build produces an APK with Android built .so libraries downloaded from TBPL.