`mNumber` maybe if this is not the interface of the real API?

I personally write these interface mocks as a initializable pattern too so I can isolated instances for every test, (see https://bugzilla.mozilla.org/show_bug.cgi?id=974319 https://bugzilla.mozilla.org/show_bug.cgi?id=987533 https://bugzilla.mozilla.org/show_bug.cgi?id=1005751), you can do that here too (not a r+ requirement here)