I lost the horizontal scroll bar a few days ago, i.e. some time after the last update. I don't know what I did. I have Firefox 78.0.1 on Windows 10. I can still scroll using the left and right arrow keys but using the mouse is much easier. I still have a vertical scroll bar and can still use the mouse to scroll vertically.

Cancel this question - the problem is with the website, not with Firefox. Other websites do not show this problem.