This is usually caused by a problem with a specific font and you need to identify this font and fix this. You may find that it can be resolved by finding and removing duplicate fonts installed in the local ~/Library/Fonts folder, that are conflicting with standard system fonts. You can check in Font Book for font issues like corrupted and duplicate fonts. Note that you shouldn't disable "Allow pages to choose their own fonts, instead of your selections above" as this will cause issues with iconic fonts used by webpages to display small icons (you may see text labels instead of icons). You may find that it can be resolved by finding and removing duplicate fonts installed in the local ~/Library/Fonts folder, that are conflicting with standard system fonts. 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/ 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>Fonts</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://firefox-source-docs.mozilla.org/devtools-user/page_inspector/how_to/edit_fonts/