That is usually a problem with a font that Fire can't access for some reason.

You may find that this 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 should be careful about disabling "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).

I looked for changes in the Fonts directory, /System/Library/Fonts directory etc. The last modified dates were all Aug 4. This issue appeared yesterday (Sep,4th)/ May be firefox was updated, not sure though. Will try restarting my deskptop later.

If you see this happen on a webpage then you can easily check in the Inspector what font this is about.

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 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 necessary expand "All fonts...". 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/

For checking fonts in the user interface you need to use the Browser Toolbox and not the Inspector, this toolbox is not enabled by default and needs to be enabled via the Settings page of the regular developer tools like the Inspector (press F1 when the inspector has focus).

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

The problem disappeared on its own. I shut down my desktop on Friday. May be that helped. Anyway, thanks a lot for the help.