Sometimes that means the required font is not installed.

Type about:preferences#content<enter> in the address bar. Across from fonts and colors, press the Advanced button. On the bottom, turn on Allow Web Sites To Choose Their Own.

Fonts Information - Detected via Flash http://browserspy.dk/fonts-flash.php?detail=1

Hello oldmacnewbrowser,

Just in case the above doesn't do the trick, would you try to lower the value of this preference : security.sandbox.content.level to maybe 3 to start with. If that doesn't change anything, try 2 and finally 1.

To do so, type in the address bar about:config and press Enter (promise to be careful, if asked)

Then type in the search bar and look for the preference :

security.sandbox.content.level

and toggle the value.

After you've changed the value, close and restart Firefox.

If this doesn't change anything, reset the value to the default ('5')

Thanks! - Setting the sandbox to 2 sorted it out. Most appreciated!

oldmacnewbrowser said

Thanks! - Setting the sandbox to 2 sorted it out. Most appreciated!

My pleasure - glad it worked for you and things are back to normal.

By the way : well done  !