In the mac settings about keyboard/shortcuts, you can define program specific shortcuts; e.g. take the exact name from the firefox menu for the 'next tab' action and give the shortcut you want.

Right, so that's part of my gripe. in keyboad shortcuts, i've got global (all applications) shortcuts for select/show next/previous tab. Those dont map to firefox. Fine, but Firefox doesn't have a menu title for next tab, so setting a shortcut for a non-named function doesn't work. I've tried the documentation(https://support.mozilla.org/en-US/kb/keyboard-shortcuts-perf...), using "go one tab to the left" doesn't work. It's just way more effort to figure out than it needs to be for a browser in 2019.....

I apologize, you are completely right (was not on the Mac when I answered and mixed it up with Terminal and Finder).