Your System Details list shows a lot of font prefs that have a setting of size 10.

You can check for problems with preferences.

Delete possible user.js and numbered prefs-##.js files and rename (or delete) the prefs.js file to reset all prefs to the default value including prefs set via user.js and prefs that are no longer supported in the current Firefox release. See also:

• http://kb.mozillazine.org/Preferences_not_saved
• http://kb.mozillazine.org/Resetting_preferences

What do you mean by my having many Font Prefs? Is there a line item somewhere for every Font family? The PREFERENCES panel has sizes for Default, Proportional, Monospace and the Minimum. None of which are 10.

I think cor-el meant to type 20 instead of 10. If you click the More System Details link to the right of your question and scroll down, you will see the numerous minimum font size settings there. They affect character sets other than "Western", which is set to 12.

While you could use the "Fonts for" selector at the top of the Fonts dialog to modify all these other character sets, it would be faster to use the about:config preferences editor, as follows:

(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 search box above the list, type or paste mini and pause while the list is filtered

(3) For the various font.minimum-size.* preferences which are set to 20, you can right-click > Reset them to clear that minimum (right-click with the right hand and r with the left hand makes this speedy).

Or if you don't want to reset all them, just edit font.minimum-size.x-unicode to 12 since that likely is the culprit.