browser.preferences.instantApply true works for me. Thx. BTW.: between Wolfgang Rosenauer's comment #6 from 06:53:24 UTC and my present reply I've checked more than 10 different workstations at our place, all running the same openSUSE 13.1, 64bit, standalone installation, with Thunderbird 38.3.0 and Lightning 4.0.3.1 ... they all show bug #941868 reported here, and they all get fixed with "browser.preferences.instantApply true"