I was hoping I could do a CTRL+Left arrow or CTRL+Right arrow in order to move the tab focus to a different tab. I would find that easier then mouse clicking.
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I know you didn't ask about an add-on, but I do have an add-on that lets you create shortcut keys for switching to tabs to the left or right. In case you it might help for now:
To change the keys to Ctrl+Left and Ctrl+Right, check the steps in this article: https://support.mozilla.org/kb/manage-extension-shortcuts-firefox