This is usually caused by the font that Firefox tries to use. For the user interface it is hard to diagnose what font this is about, but if you also have it on webpages then you can use the Inspector.

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 Rules tab in the right panel in the Inspector what font-family is used for selected text. 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.

• https://developer.mozilla.org/en-US/docs/Tools/Page_Inspector/How_to/Edit_fonts

Hi!

I believe this would be in firefox settings somewhere, as it's in the address bar and tabs, not as much on the general content of a website.

Any suggestions?

Here's another photo

For webpages you can right-click such a corrupted text and go to the Fonts tab in the Inspector to see what font this is about.

• https://developer.mozilla.org/en-US/docs/Tools/Page_Inspector/How_to/Edit_fonts

Possible culprits are an Helvetica font.

Hi! The issue is within the firefox browser, not in webpages.

Any idea?

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