First thing to check would be your zoom level. To reset that, you can use either:

• Ctrl+0 (that's a zero) or
• View menu > Zoom > Reset

If you don't normally display the full menu bar, tap the Alt key to display it.

If that works, a common way to end up in that position is to be holding the Ctrl key down when you scroll the mouse wheel.

If this nearly never happens to you, I wouldn't change how it works.

If you get annoyed by zoom changes on a regular basis, you can turn off this feature. Here's how:

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box above the list, type or paste wheel and pause while the list is filtered.

(3) Double-click mousewheel.with_control.action and enter the desired value from the following list, then click OK.

0: Do nothing (ignore the wheel)
1: Scroll the page
2: Back to previous page or forward to next page in history
3: Zoom in or out

(For Mac, users, to change what happens with Command+ScrollWheel, double-click mousewheel.with_meta.action.)