Set default zoom in Firefox
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
No zoom and layout.css.devPixelsPerPx at 1.2