This likely means that Firefox isn't able to access the requested font and falls back to a 'last resort' font that shows all glyphs as a '?' or an 'A'.

You can check for sandbox security issues when Firefox has a problem to access fonts installed locally.

• https://wiki.mozilla.org/Security/Sandbox

You can set this pref to 2 or 1 on the about:config page to reduce the sandbox security level.

• security.sandbox.content.level = 1
• close and restart Firefox to make the change effective.

If '1' still doesn't have effect then try '0' to disable the sandbox.

If this didn't work then undo/reverse the change and reset the pref via the right-click context menu to the default value.

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

See also:

• https://wiki.mozilla.org/Security/Sandbox#Content_Levels

The same thing happened to me when I upgraded my MacOS to Catalina.

I tried disabling all fonts to try to isolate a conflict and was not able to figure it out. Any help here would be appreciated as it is very annoying.

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 Font 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/Tools/Page_Inspector/How_to/View_fonts

When I hover over ".LastResort" it highlights the question mark boxes. It says what is used is Helvetica, which I have loaded on my computer, but it is not loading for some reason. Any thoughts?

This probably means that either the Helvetica font is corrupted or is installed in a location that isn't allowed (whitelisted) in Firefox.

Try to uninstall this Helvetica font via Font Book.

The system will not allow me to uninstall it. I'm getting the attached message.

Did you make sure that only this one system Helvetica font is installed and not other fonts that report as Helvetica ?

There is also Helvetica Neue installed. But I cannot disable or remove either one.

I am using FontExplorer X Pro on macOS Catalina and also having problems with fonts not displaying properly.

FontExplorer X Pro overides Font Book.

Also noticed numeral nine (9) not displaying properly in documents on the internet!

Any suggestions please as it's driving me crazy.

This started a couple of days ago. Has affected my online email as well as many websites. This issue is rendering firefox completely useless.  :-((

I'm also using a mac and tested on safari and its definitely ff.

I updated in the hopes that would fix it but no such luck. Unfortunately, I am tech-challenged (especially when it comes to macs) & don't feel comfortable trying any of the above suggestions.

If anyone has any updated info, I would appreciate some help.

Sorry it's been months and even after an OSX update I still getting the same problem! So unfortunately I have had to switch to using Safari which is sad.

Hope you get a reply from someone…

Thank you for replying helen. It's really sad & disappointing how mozilla powers that be have allowed firefox to deteriorate over the years with seemingly no regard for their users.

The sandbox security level fix worked for me, and I'm no techie but it was pretty straightforward. Thanks!

Thanks jbuckingham. I went to the first link and my brain immediately glazed over. :p)

Then I saw these instructions:

"You can set this pref to 2 or 1 on the about:config page to reduce the sandbox security level.

   security.sandbox.content.level = 1
   close and restart Firefox to make the change effective. 

If '1' still doesn't have effect then try '0' to disable the sandbox.

If this didn't work then undo/reverse the change and reset the pref via the right-click context menu to the default value.

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue."

Is that what you did?

Hi CallyCat

Did you try to check the font in the Inspector ?

Then at least you know what font this is about.

Are you using an external font manager that might be installing fonts in another location ?

You can right-click and select "Inspect Element" to open the builtin Inspector with this element selected.

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

Thank you for your response, Cor-el. No, I didn't. Unfortunately, I know nothing about inspector, html, element button or rules pane. I cannot find any reference in the links as to where/how to access them. I'm sorry I'm so techie illiterate. If I could tell me were to find/access them, then [maybe] I could at least try to follow the directions (even tho they seem like Greek to me).  :-))

CallyCat said

Thanks jbuckingham. I went to the first link and my brain immediately glazed over. :p) Then I saw these instructions: "You can set this pref to 2 or 1 on the about:config page to reduce the sandbox security level. security.sandbox.content.level = 1 close and restart Firefox to make the change effective. If '1' still doesn't have effect then try '0' to disable the sandbox. If this didn't work then undo/reverse the change and reset the pref via the right-click context menu to the default value. You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue." Is that what you did?

Yep, that worked for me. I'm no IT expert but followed the instructions and it's been fine ever since. Good luck!

I did the Sandbox fix above and that worked! Praise The Lord!

Over the moon I followed jbuckingham's concise instructions and I'm back with Firefox -- Whoopie. This chap or chapess need a medal, thank you

Thank you so much, jbuckingham & Cor-el! Changing the sandbox thing to a 1 seems to have worked. I can read my email again AND my email emojis are once again visible.  :-)