What does MR version stand for?
I think it means Maintenance Release 1.
Source: I just heard it somewhere on a different application using MR in their version.
mr = "maintenance release", fr = "factory rom".
JBQ
(via "Kernel suffixes," Android Building mailing list)
JBQ is a former tech lead of the Android Open Source Project.
It could also mean Manufacturer's Release. As in this is the release your Manufacturer did, in this case Google. Motorola would have a completely different MR1 release in this case.