Have you tried to reset Firefox? The Reset Firefox feature can fix many issues by restoring Firefox to its factory default state while saving your essential information. Note: This will cause you to lose any Extensions, Open websites, and some Preferences.

To Reset Firefox do the following:

1. Go to Firefox > Help > Troubleshooting Information.
2. Click the "Reset Firefox" button.
3. Firefox will close and reset. After Firefox is done, it will show a window with the information that is imported. Click Finish.
4. Firefox will open with all factory defaults applied.

Further information can be found in the Refresh Firefox - reset add-ons and settings article.

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Firefox now adjusts the page zoom level according to your Windows settings, to better supporting high DPI displays. If you have a medium adjustment (e.g., 125% font size / 120dpi) you might get by with a negative zoom (Ctrl-) on those few pages with generously sized text. If you have a large adjustment (e.g., 150% font size / 144dpi), this definitely is going to be inconvenient.

Global Zoom Factor

The following setting governs both the content area and the "chrome" area (menus, toolbars, and tabs). You can adjust the value, but it might be hard to find just the right number:

(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 Firefox 21-sized fonts, 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.

The tricky part is that 1.0 makes the content normal-sized but generally renders the chrome area somewhere between almost unreadable and invisible.

Workarounds

If you can't find a good size compromise, consider these extensions to adjust the size of one or the other:

• Theme Font & Size Changer - for menu, toolbar, tab area
• Default FullZoom Level - for content
• NoSquint - for content

Can you get it to work the way you want?

Code changes regarding HiDPI have landed in Firefox 22 and later (bug #844604).

You can modify the layout.css.devPixelsPerPx and increase or decrease the value in 0.1 or 0.05 steps to adjust the size of fonts and other elements in Firefox.

• The layout.css.devPixelsPerPx pref is a String value parsed to a float and allows to fine tune the dimensions of all elements (user interface and web pages) more precisely (resolution 0.1 or 0.05).
• You can use values between 0.05 and 1.0 to reduce elements in size or use values greater than 1.0 to magnify and make elements larger (percentage divided by DPI, % / DPI).
• http://kb.mozillazine.org/about:config