Is this with all websites/pages, some, a few?

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

"Allow web sites to choose their own fonts" was already selected. Still having this issue.

Are you using a Font manager that installs fonts in a non default location ?

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