The sync migration process will need a utility function to determine the FxA account name to default to. There are 2 distinct cases: 1) If the user has previously migrated a different device, we want to use the account name used for that migration. Bug 1017433 will write that data on migration - this bug is the ability to read that value. 2) If there are no devices with this account name, we should try and use the "email address" associated with the legacy sync account. The "scare quotes" are to reflect that there is no strict requirement the account name is an email address - so we'd need to do a sanity check. This functionality should be built into 1 utility function - the migration process will just want to say "what address should I default to?" without any real knowledge about which of the 2 scenarios above apply. Note also that this will be a default value - the user will be able to change it - so it's reasonable for (2) to return 'null' (ie, "I don't know") - but (1) should ideally work reliably, which means it might need to perform a metadata sync to get the data. See bug 1017433 comment 2 for more context for (2) - which will probably require sync tests (so p=8)

This landed as part of the migrator module (bug 1019985)

Verified as fixed using the following environment: FF 37.0b4 OS: Mac Os X 10.9.5, Ubuntu 14.04 x64, Windows 7 x64 The email suggestion is the correct one in both cases described in comment 1 .