You can easily accidentally shrink the display of a site, using the zoom feature, by holding Ctrl while scrolling the mouse wheel. But usually that only affects the current site and not all sites. (Or there is a way to switch it so it affects the current tab but not all tabs.)

Anyway, to reset the zoom level on the current tab, you can use Ctrl+0 (that's a zero) or

View menu > Zoom > Reset

When you are using the orange Firefox button (short menus), you can tap the Alt key or press F10 to display the classic menu bar to use the old View menu.

Does that help?

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