Type about:preferences#content<enter> in the address bar. Across from fonts and colors, press the Advanced button. On the bottom, turn on Allow Web Sites To Choose Their Own.

Your zoom controls.

<Control> + (plus) enlarge <Control> - (minus) reduce <Control> 0 (zero) restore

Check that you do not run Firefox in compatibility mode or with a reduced screen resolution. You can open the Properties of the Firefox desktop shortcut via the right-click context menu and check the "Compatibility" tab. Make sure that all items are deselected in the "Compatibility" tab of the Properties window.

Try to set layout.css.devPixelsPerPx to 1.0 (default is -1) on the about:config page. If necessary adjust its value in 0.1 or 0.05 steps (1.1 or 0.9) until icons or text looks right.

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config

You can look at the Default FullZoom Level or NoSquint extension to set a default font size and page zoom on web pages.

• Default FullZoom Level: https://addons.mozilla.org/firefox/addon/default-fullzoom-level/
• NoSquint: https://addons.mozilla.org/firefox/addon/nosquint/