Reproduced with 2014-08-04-03-02-05-mozilla-central-firefox-34.0a1.en-US.linux-x86_64 (compared with the picture 512/289, which also causes this when used twice, but not once).
Interestingly, I can't reproduce this on mac. :-\
This is the bug where we have the same image on screen with two different sizes and we continually downscale to the first size, then the second size, then the first size, etc. Mac doesn't have HQ downscaling enabled so that explains comment 2. This is certainly a dupe, but I don't have a bug number handy.
(Mac doesn't need HQ downscaling cause it does a good enough job without it.)