It seems you have switched on caret browsing. The f7 key toggles it off and on. Make sure the following setting is NOT selected: Settings > General > Browsing > Always use the cursor keys to navigate within pages.

Note that if you do not want to use this feature you can set accessibility.browsewithcaret_shortcut.enabled = false on the about:config page to disable caret browsing that is part of the Firefox accessibility features.

You can open the about:config page via the location/address bar. If you get the warning page, you can click the "Accept the Risk and Continue" button.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox