Set default zoom in Firefox

layout.css.devPixelsPerPx to 1.2

To zoom pages by default in Firefox, open the about:config page via the location/address bar, and change the value of layout.css.devPixelsPerPx to fx 1.2, to zoom in 120% on all pages by default. This will also increase the size of icons and tabs, but this way you don't need an add-on for larger text on all web pages by default.

Zoom 133 percent and layout.css.devPixelsPerPx at 1.0

Zoom 133 percent and layout.css.devPixelsPerPx at 1.0

No zoom and layout.css.devPixelsPerPx at 1.2

No zoom and layout.css.devPixelsPerPx at 1.2