see my reply in https://support.mozilla.com/en-US/questions/854549 for a similar question.

Some reasons why pages don't fit.

• column width is determined by a picture, you might want to change options so that zoom only applies to text
• the page has absolute dimension and wide page margins
• the page was written for Internet Explorer, but with IE9 such pages are fading away fast, because many of them wouldn't work in IE9 either.
• user zoomed page, or changed font sizes

The Babylon Toolbar 1.1.8 extension is known to cause the browser window to be shifted to the right, off the right side of the screen while leaving part of the left side of the "desktop" showing.

THANKYOU! I have now deleted this Babylon Toolbar and my screen is back to the centre again :)

If you got Windows 7, click start, control panel, display (make sure it's set to smaller (default), go to upper left click adjust resolution, advance settings, list all modes and then move up to the next higher resolution that's set for your computer. This solved my problem.