Tab Saver troch Mikhail Vyrtsev

WebExtension to manage your tabs. With this extension you can save tabs in current window and open them later.

Caveats
Current state of Firefox WebExtension Api restricts opening of privileged urls (such as file:///, chrome:, about:, etc..) so placeholder page will be opened where you can copy link to clipboard and manually paste it into the location bar. Sad.