Briar requires both parties to be online and both to add each other to even begin talking.  This makes it extremely likely that you’ll have to use out of band, or non-Briar communication to coordinate when you’ll be doing Briar.  On the other hand, Session has a model similar to Nostr with nodes for offline communication and you can initiate first contact on Session.

The main difference between Nostr and Session is that Session’s nodes are darkweb onion routed and with Session it goes to the receiver's node while as Nostr the poster picks the node.  Session is not good for forward secrecy that’s correct, but it’s #1 for censorship.

Regarding the 2 browser extensions.  I will check it out.   I see your codeberg here, looks good.  When you visit websites with Qutebrowser, does it show up to the website fingerprinting you as Chrome?