home browser screen way too large (height and width)

My home screen and all other browsing screens are so high and wide that I can't scroll down because the side slide bar is not on the screen. I have no idea how this happened! I can't find any articles that help me reduce the screen size that I can understand. There seems to be no solution option in firefox settings to adjust the screen size. The View tab does not help because it just offers zooming of the data on the screen, not the actual size of the screen.