By default Firefox and Safari on macOS behave a little weirdly when it comes to tab navigation on websites in that only a few elements on a website are tabable at all (if you are lucky). I experienced this broken behavior together with a colleague during testing the tab navigation behavior for sz.de.
Activate normal tab navigation in Firefox by going into the MacOS keyboard settings, click on “Shortcuts” at the top, and activate the “All controls” radio button at the bottom.
Activate normal tab navigation in Safari by going into the Safari settings, click on “Advanced” at the top right and activate the “Highlight any object on a web page using the tabulator” checkbox.