that would probably be caused by a Theme, and Extension, or a Style, you can check that ouit quickly by testing in safe mode (one time restart of firefox)

• "Alt+H": (Help) > "Restart with Add-ons disabled", do NOT click on any of the checkmarks as they will destroy data.

If you have scrollbar there you need to isolate the problem

If only one one site, there are a lot of creative web designers who think if you have a wide screen they should make use of all of the width.

Meanwhile other methods of scrolling up and down.

• mouse wheel
• "SpaceBar" (PageDn), "Shift+SpaceBar" (PageUp)

Yes, thank you so much!

Nicole