Using the Forget button clears all pinned tabs for the new window. Reporters have stated that doing so makes Forget an unusable feature for them.

The targeted use case for this was users who don't heavily use tabs, but want an easy mechanism to clear recent activity in their session. A literal "panic button" in its inception. :) As such it's not going to be for everyone. In particular, persisting pinned tabs could also be surprising, and is a strange UX to explain -- we'd be closing a bunch of stuff, but leaving a bunch of other stuff, especially if there are multiple windows with and without pinned tabs. I'm going to call this WONTFIX, but perhaps Philipp can consider something like this for future revisions of the feature.

Pinned tabs should definitely be restored. Same is true for sidebars, they are also closed after using Forget feature.

Closing pinned tabs when using Forget button makes it a rather useless feature. Perhaps, there should be an option to not affect pinned tabs when using Forget, in a dropdown widget that appear on Forget button click. Similar to how one can indicate forget period on that widget. I can see a situation when a user borrows his/her friend's laptop to use Firefox to do some work. And if he Forgets the session after the use he will ruin his/her friend's browsers configuration.