You can check in Font Book for font issues like corrupted and duplicate fonts. 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