Thank you for your report. I can't think of a reason that it should work differently on Mac -- adding or subtracting 1 from the active tab position should be the same. Does your window have any hidden tabs, add-on tabs, or special built-in tabs that I might not be accounting for?

Thank you for testing.

Any improvement if you assign your own shortcuts: https://support.mozilla.org/kb/manage-extension-shortcuts-firefox

(If you are testing in a private window, make sure to also enable the extension to run in private windows.)