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There are two extension IDs in use, called On-AMO and Off-AMO ID. The On-AMO ID is used for any release published on AMO. As of the time I'm writing, only the v0.5 branch is offered on AMO, see issue #543. The Off-AMO ID is used for releases on GitHub. It includes an updateURL which points to update-1.x-beta.rdf. The extra, off-AMO ID is needed due to Extension Signing.
mach. In order to run custom (e.g. RequestPolicy's) xpcshell unit tests, there are two possibilities: (a) integrate the tests into mozilla-central; (b) use runxpcshelltests.py. Option (b) seemed to be much easier to me.