Cannot change theme after prefs-change

I changed in prefs.js the signed.applets.codebase_principal_support 
option to "true". Now I cannot change the theme.
I tried to change it from classic to modern in Mozilla 1.7.3 an Mozilla 
1.7.5. No Errormsg - I restarted Mozilla - no Effect!

The same on another machine under Mozilla 1.7.3. After changing 
signed.applets.codebase_principal_support I tried to change the theme 
from modern to classic. No Effect.

Also I changed back the signed.applets.codebase_principal_support option 
to "false". No Effect.

Any Idea? Is this a known bug? What can I do now? Also a Mozilla update 
don't work.