Browser’s Pixel Density

Firefox:
about:config, layout.css.devPixelsPerPx, change from -1.0 to something like 1.4 (it’s a string), will change immediately.

Chrome:
command-line switch to chrome.exe, use --force-device-scale-factor="1.2" (for example).

It may help you with larger screens (a.k.a HiDPI displays)
or simply to adjust your display for a better visibility.