You may have switched on caret browsing. *http://kb.mozillazine.org/accessibility.browsewithcaret You can press press F7 (on Mac: fn + F7) to toggle caret browsing on/off. *Tools > Options > Advanced : General: Accessibility: [ ] "Always use the cursor keys to navigate within pages" ---- *http://kb.mozillazine.org/Scrolling_with_arrow_keys_no_longer_works *http://kb.mozillazine.org/Accessibility_features_of_Firefox ---- Start Firefox in <u>[[Safe Mode|Safe Mode]]</u> to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance). *Do not click the Reset button on the Safe mode start window or otherwise make changes. *https://support.mozilla.org/kb/Safe+Mode *https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes