And here's an explanation of the value as well as an explanation of why it is set to false by default:
https://firefox-source-docs.mozilla.org/intl/locale.html#regional-preferences

Tl;dr:
Example: It might lead to a UI case like “Today is 24 października” in an English Firefox with Polish date formats.

In GNOME it's as simple as shown in the image: Settings -> Region and Language:

Their metadata provider system by default only has The Movie Database (TMDb) and Open Media Database (OMDb) but they got plugins for more:
https://jellyfin.org/docs/general/server/plugins/#official-plugins