How about the text of menus and toolbars: are those okay? If so, you might want to use an extension to globally modify the default page zoom level. If you change layout.css.devPixelsPerPx, that also will affect the menu/toolbar text sizing. I know of two extensions for changing the global zoom level: * [https://addons.mozilla.org/firefox/addon/default-fullzoom-level/ Default FullZoom Level] - simpler * [https://addons.mozilla.org/firefox/addon/nosquint/ NoSquint] - more features After setting your Global zoom level, you can still fine-tune the zoom on individual sites using Firefox's standard zoom feature (keyboard shortcut or mouse wheel). ---- If you need to change the layout.css.devPixelsPerPx preference you really should be able to find it... (1) In a new tab, type or paste '''about:config''' in the address bar and press Enter. Click the button promising to be careful. (2) In the filter box, type or paste '''pix''' and pause while the list is filtered (3) Double-click '''layout.css.devPixelsPerPx''' and change its value to '''1.0''' for fonts computed based on 96dpi, or choose a larger ratio for larger fonts. For example, 1.25 corresponds to 125% font size in Windows display settings. You may need to switch back and forth to other windows/tabs to see how it's affecting them. Be careful not to set this too small to read, i.e., be very cautious with values lower than 0.8.

Note that you can also use this extension to set the global zoom (layout.css.devPixelsPerPx) easily without having to go to the about:config page. *Theme Font & Size Changer: https://addons.mozilla.org/firefox/addon/theme-font-size-changer/