We have seen reports that this can be caused by a problem with fonts like Helvetica you have installed locally and that might be corrupted or are missing variants. Also make sure you allow pages to choose their own fonts and that you do not block remote (downloadable) fonts. *[/questions/1325619] Weird font on some website's *[/questions/1330670] Render letters ---- You can right-click and select "Inspect Element" 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

It was problem with fonts in system MacOS. I fixed it in Font Book. I removed duplicate fonts in type Helvetica. Thank you very much for your quick help.