There is probably some other reason fonts don't "show up". The following preferences affect visibility and the default preference is to allow all fonts. layout.css.font-visibility.standard layout.css.font-visibility.trackingprotection layout.css.font-visibility.private possible values - 1: only use base system fonts 2: use base system fonts + Firefox lang pack fonts 3: use base system fonts + Firefox lang pack fonts + user-installed fonts (default)

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

Is this about fonts downloaded from internet via @font-face CSS rules ?

If you use uBlock Origin make sure not to "block remote fonts".

You can check the Web Console (Tools -> Web Developer) for messages about blocked content like fonts and security messages about mixed or unsafe content.

• https://firefox-source-docs.mozilla.org/devtools-user/web_console/

You can check the Network Monitor to see if content is blocked or otherwise fails to load.

• https://firefox-source-docs.mozilla.org/devtools-user/network_monitor/

If necessary use "Ctrl+F5" or "Ctrl+Shift+R" (Mac: Command+Shift+R) to reload the page and bypass the cache to generate a fresh log.

You can right-click and select "Inspect" to open the built-in Inspector with this element selected.

• https://firefox-source-docs.mozilla.org/devtools-user/page_inspector/how_to/open_the_inspector/
• https://firefox-source-docs.mozilla.org/devtools-user/page_inspector/how_to/examine_and_edit_html/

You can check in the Rules tab in the right panel in the Inspector what font-family is used for selected text. You can check in the Fonts tab in the right panel in the Inspector what font is actually used because Firefox might be using a different font than specified by the website. If the Fonts tab is hidden, click the down arrow at the right end.

• https://firefox-source-docs.mozilla.org/devtools-user/page_inspector/how_to/edit_fonts/

Tracking Protection set to Strict or Custom has fingerprinting protection which blocks user-installed fonts.