Thanks!

This did not work at the first try, despite rebooting. However, I tried it again today, and it works now. There have been Yast and Firefox updates in between, but it is mysterious why I did not work immediately!
(Ok, more likely I am just an idiot, but I only edited the file once and tried a few times in the days thereafter, rebooting several times a day (each time I dock/undock the laptop)).