The bar across the top (has "View recent browsing across Windows and devices" at the left end) has far too many remembered tabs. It's only happening on this platform (my wife's) and not on mine. Both running Windows 10, although I'm switching us both to newer platforms because of Windows 11.

There must be some way to turn off that bar in this platform, but I can't understand all the Firefox on-line help documents, because there's no place where there is a diagram indicating what that bar is called.