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

You can check for sandbox security related issues.

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

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

Thanks for the suggestions but neither of those worked. "Allow Web Sites To Choose Their Own" was already ticked (but I tried un-ticking as well, then ticking again just to try it out) and in "security.sandbox.content.level" I tried everything from 4 to 0 and it didn't make any difference unfortunately.

hi, according to other users with the same issue, disabling hardware acceleration helps: Firefox's performance settings (requires a restart of the browser)

That seems to have done the trick! So far no missing text. Thanks for your help Philipp. :-)