I've got a rather annoying problem with my Firefox browser (51.0.1 x32) when using my full resolution 1920x1080 and layout.css.devPixelsPerPx set to -1 (auto). Try running this code in the JS console:
var mywin = window.open("about:blank",
"mywin",
"width=300,left="+(screen.availWidth - 300)+",top=0");
Works fine, right? Now go back to the JS console and run this code:
mywin.moveTo(mywin.screenX,0);
The window shouldn't have moved, but with Firefox it moves about 400 px to the left. I'm guessing because moveTo does not respect the pixel ratio. window.screen.availWidth gives me 1536, multiplied by window.devicePixelRatio of 1.25 gives 1920. If i try to move the window in X direction to anything beyond 1536 it won't do anything, because it's "too far".
Running the same code in flippin Internet Explorer gives the expected result. On Firefox it only works correctly if I set layout.css.devPixelsPerPx to 1.0 but then the Firefox UI is very small.
Can anybody else confirm this behaviour or is my Firefox / Windows messed up? I've de- and reinstalled Firefox about 5 times now.
This was a bug that existed until Firefox version 52. It was fixed in Firefox 53 after I reported it on bugzilla and now no longer exists.