Maybe it is obvious, but you can of course use drag&drop to add feed subscriptions to Liferea.
You can drag
- into the subscription list
- onto the tray icon
and Liferea will create a new subscription.
You can also Drag&Drop any web sites URL and Liferea will try to auto-discover the feed URL from it.
Note that dragging URLs into the item browser pane or a browser tab will just launch the URL in the internal browser.