It is currently used only by Firefox as [explained in the doc](https://karma-runner.github.io/1.0/config/configuration-file.html) and [Firefox is removing it internally](https://bugzilla.mozilla.org/show_bug.cgi?id=1417844), so it doesn't make much sense to keep the option around for too long
It is currently used only by Firefox as explained in the doc and Firefox is removing it internally, so it doesn't make much sense to keep the option around for too long