Since v47, per-monitor DPI worked correctly in Firefox: when moving from a high-DPI to a low-DPI screen, the UI got rescaled to match the new screen's DPI. In Firefox v52 this is no longer the case: Firefox starts in high-DPI resolution (presumably because that is my primary screen?) and stays that way when moved to the low-DPI screen. This makes it completely unusable on the low-DPI screen, because not only are the web pages too large (easily fixed with zoom), but so is all of the chrome too. Tested using a clean profile in 52.0.

I can't seem to reproduce this on my Windows 10 system (with high-dpi primary and low-dpi secondary screens); tried with Firefox 52.0, 53beta, and 55nightly, and in all cases the window re-scales as expected when moved between the screens. Could you please attach the about:support details from your system, to help us try to understand what might be going on?

about:support attached as requested. Thanks for looking into this.

Thanks for that. Unfortunately, I don't see anything there that strikes me as suspicious.... aside from different graphics cards, it's very similar to my Windows system where I'm not experiencing this bug. Any of the cc'd folk, can you reproduce this?

Could it be my weird screen layout? I don't think that info is in about:support: - Primary screen 2560x1440 at logical 0,0 and 150% DPI - Secondary screen 1024x1280 at logical -1024,73 and 100% DPI

I just tried to reproduce this issue using Firefox 52.0 and latest Nightly 55.0a1 on Windows 10 64bit but with no success... Firefox scales fine while switching through monitors. Setup I used: - Primary screen 2560x1440 150% DPI (Dell P2415Q that supports up to max 3840x2160) - Secondary screen 1024x1280 100% DPI (BenQ FP75G that supports up to max 1024x1280) (not sure what does logical -1024,73 mean, I assume it's the position of the screen, if yes, I tried with the screen on to the right, left, up, down of the primary)

Thanks. Weird. My install remains broken but I guess there isn't much else you can do. Hopefully it's just me then...