After switching to Brotli compression in bug 1352595, there's no easy way to edit the content of omni.ja files. Before, it used to be possible to use tools like 7zip or unzip to hack on the browser. I personally used omni.ja files to do quick tests that don't require a full build system: * Making local changes to localization files before committing them to repository, either to test length issues or errors. * Minor changes to XHTML or JS file that don't affect tests.
You can already unpack a full firefox directory with mach python toolkit/mozapps/installer/unpack.py (but you need to at least build $objdir/dist/host/bin/bro, which you can do with something like mach configure ; mach build pre-export mozilla-config.h modules/brotli/host) Also note that bug 1352595 is not riding the trains yet.
I should add: unpack.py does take care of chrome.manifest files, so that the resulting directory contains a working firefox without any omni.ja.
This method requires having a full clone of mozilla-central (or mozilla-unified), and a system with the packages needed to build Firefox. I think that's too big of a requirement for someone who just wants to tinker with omni.ja Side note: this needs updates https://developer.mozilla.org/en-US/docs/Mozilla/About_omni.ja_(formerly_omni.jar)
I just quickly ran some tests, and for now, I can use unzip for the use-cases I have. Those mostly revolve around debugging and inspecting the results of l10n repacks. And as those don't create brotlis yet, unzip -p of those just works.
Bulk change to priority: https://bugzilla.mozilla.org/show_bug.cgi?id=1399258