Create Firefox font package

[KrasnayaPloshchad]

Firefox is allowed to load fonts from extension, on AMO site there are some extensions contains font files to allow users that do not already have the required fonts installed on their systems, with such add-on any page which specified using Ponomar Unicode or Monomakh Unicode (or you have written specific user style) will be proper rendered on both desktop and mobile devices. One notable example is here: MathML-fonts.

You can get documents from MDN: https://developer.mozilla.org/en-US/Add-ons

[typiconman]

Fonts can be loaded using CSS. What's the point of loading fonts in an extension? I don't understand.

[KrasnayaPloshchad]

There are two key points of loading fonts in an extension. First, you can write a CSS file, which contains some @font-face rules to specify which fonts included in extension can be used. For example, the MathML-fonts extension has a CSS file including following rules:

@font-face {
    font-family: Latin Modern Math;
    src: local('Latin Modern Math'), local('LatinModernMath-Regular'), url('fonts/LatinModern/latinmodern-math.woff2');
}

Second, you need prepare font files to pack, WOFF formats would be better choice. In an extension these fonts should better put into specific folder.
You can also try to see its strucure directly via Extension source viewer which allow to see the source of any extension.

[typiconman]

OK, but what are the benefits of this over using embedded fonts (which is what we do now)?

[KrasnayaPloshchad]

This extension has benifits especially in Firefox for Android, after installation, this extension would help to some pages which used your fonts your fonts but not embedded them to make them render correctly. For example: https://en.m.wikipedia.org/wiki/O_(Cyrillic)

[typiconman]

I'll put this in the "some day" category.

[KrasnayaPloshchad]

Fonts can be loaded using CSS. What's the point of loading fonts in an extension? I don't understand.

Now you can see their source code via an online source if you want to know about, then you will see the good point.
https://robwu.nl/crxviewer/?crx=https%3A%2F%2Faddons.mozilla.org%2Ffirefox%2Fdownloads%2Ffile%2F630669%2Fmathml_fonts-3.0-an%2Bfx.xpi%3Fsrc%3Ddp-btn-primary