AutoHiDPI

Warning

Obsolete. This add-on no longer works and will not be updated.

If you used two monitors with different pixel densities — say a sharp HiDPI (Retina) screen next to an older, lower-resolution one — Firefox would look correctly sized on one but too big or too small on the other. This add-on fixed that by automatically adjusting Firefox's scaling to match whichever screen its window was on, so it always looked right. (Under the hood, it set Firefox's layout.css.devPixelsPerPx value per screen.)

At its peak in 2016 it had around 1,500 average daily users, with 27 user reviews and a rating of roughly 4.4, before it was removed from addons.mozilla.org in 2018.

It stopped working in Firefox 57 (2017). It was built on Mozilla's Add-on SDK ("jetpack"), which Firefox removed when it switched to WebExtensions.

• A WebExtensions port isn't possible. The add-on worked by setting the privileged layout.css.devPixelsPerPx preference per monitor. Mozilla declined to expose this to WebExtensions (bug 1373607, WONTFIX), so the core mechanism can't be rebuilt as a modern extension.
• Firefox now handles mixed-DPI multi-monitor setups without it on most platforms. On Windows, per-monitor DPI is native and the window rescales across monitors automatically (bug 890156, fixed in Firefox 47). On Linux/Wayland, per-monitor scaling is delegated to the compositor — the approach Mozilla now recommends.
• Linux/X11 still lacks per-monitor scaling (bug 1228570; bug 1679335 was closed WONTFIX with the guidance to use the Wayland backend). This is an X11 limitation that an extension can't fix. Switching to a Wayland session resolves it.

Thanks to everyone who used and contributed to this project!