Could you check what that font is using the Font Inspector -- here's how:

right-click (or Ctrl+click) the funky text and choose Inspect from the context menu.

Firefox should open a developer tools panel with that portion of the page's HTML code highlighted. On the right side, there will be one or two panes. On the right-most pane, if you see Fonts as an option after Layout Computed Changes (etc.), click Fonts. If you don't, click the tiny triangle at the end of that row and choose Fonts from the menu.

Then Firefox should tell you what font the site wants it to use and what it actually is using. For example:

Is your Firefox substituting some other font for Arial, or does it think it is using Arial?

Thank you, I check the font is Helvetica

I've found the solution, select settings, search for fonts and remove the tick at the bottom "Allow pages to choose their own fonts, instead of your selections above"

Thank you very much

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 see little boxes with the hex code of the character or text labels if you do not have a font installed that covers this Unicode range or block downloadable fonts or if you do not allow pages to choose their own fonts.

You should identify what font is causing and instead fix this.

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

Did you check what font is causing this issue ?

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://firefox-source-docs.mozilla.org/devtools-user/page_inspector/how_to/edit_fonts/