Since a update a few weeks ago Firefox doesn't seem to keep settings done in about:config. I noticed this for example because I use a changed dpi setting
layout.css.devPixelsPerPx=1.5
After an update this was reset to -1.0. Changing this to 1.5 again however works only temporarily. After restarting Firefox the setting is gone.
What may be wrong here and how can I fix it?