This can happen when Firefox has a problem to access locally installed fonts because of sandbox security issues. https://wiki.mozilla.org/Security/Sandbox

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.

[v56+] Type about:preferences#general<enter> in the address bar. Under fonts and colors, press the Advanced button. Scroll to the bottom, turn on Allow Web Sites To Choose Their Own.

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

after following your instructions, i restarted first the browser, then my computer, but without the expected results

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

• 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.

• http://kb.mozillazine.org/about:config