Here is a Mozilla support article containing all of the Firefox keyboard shortcuts.

See also "Help > Keyboard Shortcuts" which opens this same KB article

I have checked the documents mentioned here as well as general internet searching for similar shortcut lists. They do not contain a shortcut for this function.

It seems like something that would be in place, but maybe not.

You can try the Customizable Shortcuts add-on.

hello, crtl+tab or ctrl+shift+tab should work fine for your purpose in principle. can you try to replicate this behaviour when you launch firefox in safe mode once? if not, maybe an addon is interfering here...

Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

You can also use "Ctrl + Page Up" and "Ctrl+ Page Down" to go to the previous and next tab.

Note that keyboard shortcuts may not be working if a plugin has focus and in such a case you will have to use the mouse to move the focus elsewhere.

Thanks Phillip and cor-el,

By starting Firefox in Safe mode, I found that I must have some add ons which were interfering with the way ctrl-pg up and ctrl-pg down were working. I'll have to troubleshoot to find out which add on was causing the issue.

I had mistakenly thought that these shortcuts, described as "Previous Tab" and "Next Tab" would have toggled the tabs last viewed and next viewed. That's not correct. It's really left adjacent and right adjacent, which is what I wanted.