Report ID 766096986: API version not specified
Reports API version not specified - based on last successful sync time, this started when I upgraded Firefox nightly from 13.0a1 (2012-02-25) to 13.0a1 (2012-02-29)
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For anyone searching this - the easy way to "go back to an earlier version" is to use the Mozilla Profile Manager to run a different version of FF against the problem profile, and get the sync to occur without loss of data.