This is automatic in Firefox. When giving e.g. microphone permission, you can leave the 'Remember this desxision' option unchecked. This will make the permission 'Allowed temporarily' and will be reset when you close the tab or window.
As far as I can test, Chrome does not have this option.
Signal is just as easy to use as WhatsApp for example. You register with your phone number and you can call/text anyone on your contacts who has installed Signal.
It even has video calls and recently added screensharing to them.
The attacker can be a man-in-the-middle and make the victim authorize a costly transaction (say $100), with the victim seeing a cost of $1.
These is an example video here: https://blog.cmpxchg8b.com/2020/07/you-dont-need-sms-2fa.htm...
Of course, the real transaction can be seen from the bank.