This happens for me if the Firefox window is beyond a specific width and the layout changes from mobile layout to desktop layout and the three bar menu button is replaced by an horizontal menu bar with labels. You can verify that by making the Firefox window smaller or by zooming the page. So this is caused by the style sheets that are used and the @media rules for various screen widths.

Yes, changing the window size fixes the problem for the page I'm on, but when I go to another page, I have to resize the window again to get it to display properly. I haven't made any changes to the site's style sheets, so don't know why this is happening all of a sudden. Maybe there was a Wordpress update? I'll check with our webhost. I didn't set up the style sheets and really don't know much about them. Thank you, core-el.

Yes, same problem here with Firefox 84.0.2 on a Mac platform (Big Sur). Just recently, Wordpress top menu does not "stick" while scrolling down. This is the only browser with this issue. I tried resizing the page, then refreshing the page... but it only works temporarily. Some weird interaction with Firefox.

music9 - We use Divi themes for our website. My web guy says there is a setting in Divi themes to turn Fixed (“sticky”) Menus on or off. Turning them off fixed my problem. Perhaps changing that setting in whatever theme you use will fix yours as well. Good luck.