Yes, that's called "caret browsing" and you can turn it off on the Preferences page here:

• Mac, either:
  • "3-bar" menu button > Preferences > Advanced > General mini-tab
  • Firefox menu > Preferences > Advanced > General mini-tab

• Windows, either:
  • "3-bar" menu button > Options > Advanced > General mini-tab
  • Tools menu > Options > Advanced > General mini-tab

Uncheck the box for "Always use the cursor keys to navigate within pages"

Success?

debway said

I somehow pressed some key combination on my keyboard that now always puts a cursor in the content of my Internet browser page. And now when I try to use my arrow keys on the keyboard to scroll down email, content, etc., it jumps to the top of the page instead of scrolling. How can I fix this?

Thanks! I did find that earlier and noticed it worked. Thanks for reinforcing that!

Caret Out {web link} Disables change of the caret browse mode by shortcut key(F7).

You can toggle this pref to false on the about:config page to disable the F7 shortcut if you do not intent to use caret browsing mode.

• accessibility.browsewithcaret_shortcut.enabled = false