Unlike on every other platform, Firefox *for Android* seems to do a special rounding behavior of button border corners, even for unthemed buttons (i.e. buttons where the author has specified a custom background and/or border). This caused a decent amount of extra work (to diagnose/address an Android-only reftest orange) when I was to get Mihir's patch for bug 1397768 landed. (In the reftests in that bug's initial patch, there were various borders/backgrounds specified to help visualize different descendants of a button, and the key difference between reference case vs. testcase was that one file has various styles set on the button itself, whereas the other file had these styles set on a 100%-sized wrapper-div inside the button. This shouldn't have made a difference painting-wise for background/border, but it apparently does make a subtle difference right now, specifically on Android for the 4px grid at each corner of the button.) I've got a strawman reftest locally that demonstrates this issue -- I'll post that shortly, after I've confirmed that it's problematic via a Try run.

Here's a patch to add 2 reftests that demonstrate this issue. One test uses border, the other uses background -- but each of them is comparing the {border/background} being specified on a 100%-sized child of the button vs. on the button itself. Here's a multi-platform try run, showing that these reftests fail *only* on Android: https://treeherder.mozilla.org/#/jobs?repo=try&revision=93c403aab3bcab274832e1cf3ee23ebf1c2528ff&selectedJob=194943678

For posterity, here's the relevant snippet of the Android reftest log, suitable for pasting into https://hg.mozilla.org/mozilla-central/raw-file/tip/layout/tools/reftest/reftest-analyzer.xhtml