Do you think it's worth adding a second pref to the test, to ensure that the logic for appending prefs is working? I'm not sure that we need a second one that can also be tested, perhaps just adding a second one above this one would be enough to suggest that the loading of prefs is working. Locally, to test, I added:

`'--firefoxpref=extensions.checkCompatibility.nightly:false',`

above this line and the test still passes, which is good.