Make sure you allow pages to choose their own fonts. *Settings -> General: Fonts & Colors -> Advanced<br>[X] "Allow pages to choose their own fonts, instead of your selections above" This can be caused by a problem with fonts you have installed locally and that might be corrupted or are missing variants. You will have to identify and remove corrupted font(s). You can right-click and select "Inspect" to open the built-in Inspector with this element selected. *https://developer.mozilla.org/en-US/docs/Tools/Page_Inspector/How_to/Examine_and_edit_HTML You can check in the <b>Rules</b> tab in the right panel in the Inspector what font-family is used for selected text. You can check in the <b>Font</b> 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. *https://developer.mozilla.org/en-US/docs/Tools/Page_Inspector/How_to/Edit_fonts If you use uBlock Origin make sure not to "block remote fonts". If you use extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) that can block content (Adblock Plus, NoScript, DuckDuckGo PE, Disconnect, Ghostery, Privacy Badger, uBlock Origin) always make sure such extensions do not block content. *make sure your extensions and filters are updated to the latest version *https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems