To add them manually click on the star to the right of the senders address when viewing the message in the message pane or a tab. If the star is filled with color, the contact is already in your address book. If the star is blank, clicking it will add it. Clicking the star a second time will open the edit dialog box.

This is NOT the same star that is to the left of the message header in the Inbox view.

There are some add ons that will add all incoming addresses to their address book. I cannot imaging why anyone would want to do this, but it is available. You can search the add on list for these applications if that interests you.

Automatically? Really?? All addresses of all senders just sucked into the address book, spammers, advertisers and all?

Where an address appears in the panel above the message text, there is a star alongside it. Click this star to add the address to your Address Book. Once you have done that, you can click the star again to edit the address book entry.

Note that if the star is empty/grey (silver?) the contact is unknown. If the star is filled/yellow (gold?), the Contact is already in your address book. (This is very like the way bookmarks are managed in Firefox.)

If you really want all senders' addresses entered indiscriminately into your address book, this add-on may be of interest.

https://nic-nac-project.org/~kaosmos/index-en.html#addrcollector