That’s interesting.
None of my tabs ended up at the bottom or on the side. Everything stayed intact when I updated to the latest version.
By tabs on bottom, I mean beneath the address bar, not at the bottom of the screen. That was the original design of Netscape then Firefox and you had your choice with an easy change with about:config. Now to continue to have it look that way, you have to be a programmer (which I am not) to write code for the userChrome CSS file. It all just sucks.