There is one exceptional case in which the webfonts cannot be downloaded automatically: when Firefox is used to read documents in a local file system (as opposed to reading them from an HTTP server). In this case, Firefox's strict same-origin policy regarding the use of webfonts prevents the download.
- Workaround
To read math with Firefox, you need to install the MathJax TeX Open Type fonts on the local computer. Installing the appropriate fonts is straightforward. The fonts are the Open Type MathJax TeX fonts found in:
<doc_install_dir>/Doc/English/MathJax/fonts/HTML-CSS/TeX/otf/
where <doc_install_dir>
is the root directory in which the documentation has been installed.
With appropriate user privileges (e.g., by running as Administrator), the files should be copied to one of the computer's font directories and installed as required by the operating system.
For example, on Windows the font directory is C:\Windows\Fonts\ and to install any or all of the fonts, just right-click a selection and choose Install.