Hi

Could you tell us what that key icon did?

I don't see it present in the UI or in the Customize tab by default.

Could it be an addon which is now disabled?

The key icon opened the password manager. The only way I can open it now is through preferences, so it cannot read new sites to store logins

This is what you are talking about;

https://addons.mozilla.org/en-US/firefox/addon/saved-password-editor/ Saved Password Editor Adds the ability to create and edit entries in the password manager.

You were absolutely right. It's back in all its simplicity. I wonder why it left? But that's just curiosity for curiosity's sake.

I'm very grateful

Glad to help. Safe Surfing.