do yo log out from the site? try resetting your browser or reinstall may fix the problem.

Resetting the browser did not help. I don't believe that after 20+ years with Mozilla (my first browser was Mosaic) I have to migrate away to Chrome. :(

If the tab has no previous history, the back button will be disabled. That is normal.

Quote: If I go to Facebook from a blank new tab

If you first open a blank tab and then open a web page in this tab then there is no tab history for this tab. You can close the tab to go back to the previous tab. If you would open the page from an existing tab then you can close the tab. If select owner on close is selected (browser.tabs.selectOwnerOnClose = true) then you will return to the tab.

Again, it appears we have a failure of communication. Let's try again. I open a blank tab. I enter Facebook. I browse Facebook by going from page to page within Facebook. As I'm doing so, the back button remains greyed out, no matter how many Facebook pages I visit. There is no Back option. The glitch appears on other sites as well, although in a less consistent manner. The Back button does not take me to a previous page, but to the previous URL domain.

It is possible that pages are loaded in a frame or that content is loaded via JavaScript (AJAX). In such a case the page content changes, but the URL stays the same.