Librewolf not retaining language for displaying pages

This is maybe not related to NixOS itself but maybe one of you has got a suggestion:

I am using LibreWolf from unstable and everything works as intended, except it doesn’t retain my preferred languages for displaying pages after closing LibreWolf.

I can set them normally in the settings and they are applied correctly in the current session. They are also correctly written to prefs.js in the profile directory of LibreWolf.

However, when i restart LibreWolf the settings are set to default and the entry in prefs.js is overwritten by the default setting.

I tried to force LibreWolf to use my intended settings with a user.js and a autoconfig.js file but to no avail. These settings are not respected and LibreWolf always defaults to english as the default language for displaying websites.

Have the Firefox / LibreWolf users among you got an idea, what this could be about?