I have installed
FT Graphite Glow 5.11
Classic Compact 16.0.2
FT DeepDark 5.4
After about 20 or so startup or opening of Firefox, Firefox reverts back to the default appearance. Why??? This has never happened before version 17. What has changed????
Try to disable hardware acceleration in Firefox.
You can check for problems with preferences and try to rename or delete the prefs.js file and possible numbered prefs-##.js files and a possible user.js file to reset all prefs to the default values.
You may have other extensions that are interfering.