Mozilla Connect

This is tracked by https://bugzilla.mozilla.org/show_bug.cgi?id=2012210. 

Since Firefox 153 (Beta as of today) you can assign custom shortcuts for both "Add Split View" and "Separate Split View" in about:keyboard. https://bugzilla.mozilla.org/show_bug.cgi?id=2033319

Duplicate tab is planned for Firefox 151. https://bugzilla.mozilla.org/show_bug.cgi?id=2031128

Duplicate Tab is planned for Firefox 151. https://bugzilla.mozilla.org/show_bug.cgi?id=2031128

Assigning a shortcut to "Duplicate Tab" in about:keyboardt should become available with Firefox 151.https://bugzilla.mozilla.org/show_bug.cgi?id=2031128