It would be convenient at least. There's pushPrefEnv, but as far as I can tell it doesn't actually wait for the pref to be synced to the child and the nsPref:changed notification to be sent before calling its callback. See bug 1253479 for motivation. We had to set a pref from the child, and because the code under test also runs in the child, the test had to observe the pref change before continuing. Previously it had not been running under e10s, and it assumed that the code under test observed the pref change instantaneously.
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