You may have switched on caret browsing.

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

You can toggle caret browsing on/off 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

Hello,

Try Firefox Safe Mode to see if the problem goes away. Safe Mode is a troubleshooting mode, which disables most add-ons.

(If you're not using it, switch to the Default theme.)

• On Windows you can open Firefox 4.0+ in Safe Mode by holding the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac you can open Firefox 4.0+ in Safe Mode by holding the option key while starting Firefox.
• On Linux you can open Firefox 4.0+ in Safe Mode by quitting Firefox and then going to your Terminal and running: firefox -safe-mode (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)
• Or open the Help menu and click on the Restart with Add-ons Disabled... menu item while Firefox is running.

Once you get the pop-up, just select "'Start in Safe Mode"

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, and you need to figure out which one. Please follow the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article for that.

To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

When you figure out what's causing your issues, please let us know. It might help other users who have the same problem.

Thank you.

It likely is caret browsing though