Example log: http://ftp.mozilla.org/pub/mozilla.org/b2g/tinderbox-builds/cedar-linux32_gecko/1391170408/cedar_ubuntu32_vm-b2gdt_test-reftest-1-bm53-tests1-linux-build0.txt.gz These tests are also failing on the emulator (bug 783632), but it looks like a separate problem. There the images were being rendered in the top left of the canvas instead of the centre. Here the tests are failing due to actual differing pixels.
It looks like the gamma is off. Seems likely to be straightforward to fix.
We're dropping b2g desktop support in reftest, see bug 1226017