With recent versions of firefox, the prefs.js file automatically written when firefox exit now contains the header: # Mozilla User Preferences /* Do not edit this file. * * If you make changes to this file while the application is running, * the changes will be overwritten when the application exits. * * To make a manual change to preferences, you can visit the URL about:config */ This break mozprofile, which is not able to read this and raises a Syntax error - therefore mozregression says "Unable to start the application", print the stack trace and skip the build. This should be fixed upstream with bug 1233534. We just need to update the required mozprofile version here.
This is an easy change - I landed it in https://github.com/mozilla/mozregression/commit/8bc8dd84e238297519e3f37859b80d62b028bc17.