Firefox 30 not recognizing font

If you want to override the fonts in Wikipedia specifically, edit Special:MyPage/vector.css (for the default Vector skin):

Example:

#bodyContent {
    font-family: serif;
}

This is stored in your Wikipedia account and works for all browsers. If you are using a different skin (theme), change the .css name (e.g. monobook.css).

(I assume you meant to write "display in serif instead of sans", because "sans" and "sans-serif" are the same thing.)

Example for a specific font: font-family: Garamond, "Times New Roman", serif;

For a more general solution, install the Stylish extension for Firefox or Chrome. (In Firefox, you can manually edit userContent.css.) This way you can set overrides for specific sites, or for everything.

Example for Wikipedia:

@namespace url(http://www.w3.org/1999/xhtml);
@-moz-document url-prefix(http://en.wikipedia.org/) {
    #bodyContent {
        font-family: serif;
    }
}

It's mostly the same as above, but with added @namespace, and wrapped inside an @-moz-document block.

I had the same problem for quite some time and GitHub was a pain to watch. I finally installed fontconfig-ttf-ms-fonts today and problem solved!