try via this link. https://addons.mozilla.org/en-US/firefox/addon/dark-mode-webextension/

even if its not able to add, make sure you have default settings in firefox

This is not the problem nor the good extension

1.Open the Chrome Web Store.

   2.Find and select the app or extension you want.
   3.Look at the URL. The ID is the long string of characters at the end of the URL. For example, nckgahadagoaajjgafhacjanaoiihapd is the ID for Google Hangouts.

The extension ID are the same in FF and Ch ?

Because in FF UBO have something like "support@gorhill" and Ch have "cjpalhdlnbpafiamejdnhcphjbkeiagm"