The ability to display the names of buttons in Firefox was removed at some point in the past. That functionality can be restored using “Classic Theme Restorer (Customize UI)” by Aris.
Click green “Add to Firefox”. Wait for installation dialog to appear, then click “Install Now” if you choose to trust the author.
Restart Firefox, as requested.
You can new see the theme has changed. The tab is back to the better, in my opinion, boxy style.
Click the “Open menu” icon, the button on the far right with 3 bars, also called a hamburger. This will expand the menu. Click “Customize” at the bottom.
Near the bottom is “Mode”. Select “Icons+Text” or just “Text” depending on your preference. You will see a live preview. Click the green “Exit Customize” when done.
That’s it. All back to the way it was.