If you use mach to build Firefox, there is a chance you may run into some errors from a change that landed on mozilla-central Wednesday.
I reimplemented the mozconfig finding and loading logic in Python and with a nice shell wrapper that lets us more easily inspect side-effects of execution. The overall goal is to speed up mozconfig loading (fewer shell invocations) as well as to have a strong API for interacting with mozconfigs so we can do things like better validation. As a side effect, we now have test coverage of mozconfig behavior!
Anyway, because the logic for mozconfig finding and loading is going through a completely new mechanism, you may notice some weird behavior. Until a few minutes ago, $topsrcdir wasn't defined in the execution environment. This is now fixed. I think most complaints stemmed from this.
Another significant change is that mozconfig shell scripts are now executed in a shell that has set -e. This means that failing commands in your mozconfig will now abort execution. Before, execution would have silently continued unless you checked the exit code in your mozconfig. I think this is a change for the better, but it may break your mozconfigs.
If you encounter an issue, please follow the instructions in mach's output to file a bug. Please attach or link to your mozconfig.