Hey guy 
Guess need to ask a few questions, and suppose that you are talking about scrolling from side to side, horizontally:
resolution setting?
monitor size?
suppose that you have your browser window maximized?
does this happen at all sites that you visit?
with all browsers [safari and/or Firefox, etc.]?
Click the bright blue apple at top left...
Click system prefs...
Click on displays [hardware, 2nd row]
Click on display - what is the res selected?
Guess need to ask a few questions, and suppose that you are talking about scrolling from side to side, horizontally:
resolution setting?
monitor size?
suppose that you have your browser window maximized?
does this happen at all sites that you visit?
with all browsers [safari and/or Firefox, etc.]?
Click the bright blue apple at top left...
Click system prefs...
Click on displays [hardware, 2nd row]
Click on display - what is the res selected?