Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

• Do NOT click the Reset button on the Safe Mode start window or otherwise make changes.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

You can also check for problems with the localstore.rdf file.

• http://kb.mozillazine.org/Corrupt_localstore.rdf

I restarted FF in safe mode and the icon appeared. I then restarted FF to see about selectively shutting down add-ons to see if I could narrow down the problem. When I restarted FF the download icon was visible, although I didn't make any changes.

Starting once in Safe Mode may have caused Firefox to recreate a new localstore.rdf file that worked.