On a three-button mouse, clicking the middle button turns the mouse into a scroll controller. Try moving your mouse slowly.

I haven't turned on the scroll wheel. I never touch it. I don't move my mouse at all when the page starts just randomly scrolling up and down. I'm not even touching it when this happens. It's almost as if there's someone else controlling my computer from another computer, like those connected to a common server in a workplace.

That is possible.

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. Is the problem still there?

You may have switched on caret browsing.

• http://kb.mozillazine.org/accessibility.browsewithcaret

You can toggle caret browsing off/on by pressing F7 (Mac: fn + F7).

• Tools > Options > Advanced > General > Accessibility: [ ] "Always use the cursor keys to navigate within pages"

Note that this is an accessibility feature of Firefox.

• http://kb.mozillazine.org/Scrolling_with_arrow_keys_no_longer_works
• http://kb.mozillazine.org/Accessibility_features_of_Firefox

I can't start Firefox with my shift key down. It won't start up like that. The auto scrolling only occurs after I've been online for a time. As for the scrolling with the cursor key in the advanced options, it's never been checked. I don't know why it does that. It doesn't just go one direction, either. It'll go up and down, almost as if someone is searching for something.

Were you doing anything at the time, pressing keys? Do you have a mouse pad? Go into the computer settings and have the pad disabled when a mouse is detacted.

Type about:support<enter> in the address bar. At the upper right, press the Restart With Add-ons Disabled button.