Nix / NixOS

Hi all! I'm new to NixOS and so far I'm loving it! I'm in the middle of the process of "nixifying" all my stuff, and I've found a problem that I've not been able to solve yet: When using LibreWolf on my Mac OS laptop, I can easily add exceptions to store cookies from the padlock icon in the search bar, as you can see in the screenshot, so I don't have to re-login to some sites every time I open my browser. The problem is that on my NixOS laptop there is not such an option. I have compared settings in both setups, and they both look the same. I even tried explicitly setting pref.privacy.disable_button.cookie_exceptions to false, but no luck. The option is still not showing in the padlock menu. Here's my LibrefWolf config, in case you are curious.

I know I can manually add exceptions in the settings page, but doing it directly from the search bar is way more convenient.

I searched all around the interwebs before writing this post, but I found nothing. Does anyone know how can I enable this option? Thanks a lot!