Ctrl plus cursor left and right keys will move the current tab to the left or right on the tab bar, leaving the focus on that tab, if the tab bar has focus and shows a focus ring.
You can use Ctrl+K or Ctrl+L and press the tab key once or twice to set the focus to the tab bar (you can't do that by clicking the tab on the tab bar).

Ctrl + (Shift+) Tab moves the focus to the next or previous tab, the same as Ctrl + Page Down and Ctrl + Page Up. That is an entirely different action.

Thank you, sir/ma'am. If the mouse is on the tab bar, then one would more likely move the mouse & not the keyboard, so I regard your reply as almost irrelevant (sorry, sir/ma'am). The only advantage to using the keyboard to do this would be to move left at the extreme left (off the screen) or or move right at the extreme right (also off the screen). The Ctrl Tab combos are therefore more useful. BTW, what's a focus ring? Furthermore, the page in question DID NOT give the requirements as you stated it, nor did it list the Ctrl+tab commands. May I assume the Ctrl+tab commands are universal without the qualifications you gave for the originals?

The point is that you are reordering tabs via Ctrl Left and Ctrl Right by moving the current tab to another position on the tab bar.
The focus ring shows as a dotted square around the text on the tab if the tab has focus.
Press Ctrl+K and press the Tab key once should set focus to the Tab bar.
If you only want to switch to another tab and not change the ordering then you need to use the other keyboard shortcuts Ctrl+Tab.

I understand your points 1 & 3, but not "The focus ring shows as a dotted square around the text on the tab if the tab has focus. Press Ctrl+K and press the Tab key once should set focus to the Tab bar." I tried that & the focus moved to what Firefox calls the navigation bar & focused on the link to another tab. Maybe I'll try it again tomorrow. BTW, I haven't seen the dotted square yet?.

You will have to disable Tabs on Top temporarily to have the Tab bar below the Navigation Toolbar to make this work.
You may have to press the tab key more than once if you have other toolbars with focusable elements between the Navigation Toolbar and the Tab bar.

Well sir, you are smarter than I am, because I did not understand your 2 sentences above. I don't know where I would "disable Tabs on Top" & what to "make" "work"? On the 2nd, besides the title bar, which is non-negotiable I have the menu bar, navigation toolbar & tab bar & finally the add-on bar at the bottom.