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

caret browsing was not and is not selected. However, nosing around, I selected properties on desktop, advanced, and slowed the hardware accelerator a notch or two...seems to have solved the problem...at least for now.