Problem
Here is a part of the screen shot of https://ruby.github.io/rdoc/.

The color of texts in a code block is black, so we cannot read it.
Cause
Under certain conditions, Firefox cannot render Source Code Pro font correctly: it ignores specified font color.
https://bugzilla.mozilla.org/show_bug.cgi?id=1520157
To solve it, Adobe released an new version of Source Code Pro:
adobe-fonts/source-code-pro#217 (comment)
Although the bug of Firefox still remain, the new version of Source Code Pro will work as expected.
Solution
The version of Source Code Pro used in rdoc is too old.
Replacing it with the new version will resolve this issue!
https://github.com/adobe-fonts/source-code-pro/releases