<i>'''WARNING from the moderator team:''' Changing Firefox preferences from the about:config page can sometimes break Firefox or cause strange behavior. You should only do this if you know what you're doing. Please read [[Firefox advanced customization and configuration options]] to learn more. </i> This is the best advice for getting rid of it the 'Official' way: https://support.mozilla.org/en-US/questions/1476477#answer-1690886 However a lot of people seem to be reporting that it doesn't work for them so, <i>if needed</i>, an alternative method is:— Type <code>about:config</code> into the search/URL bar. That will bring up a warning which you'll have to acknowledge as care needs to be taken from here. On the next screen there's a search box at the top. Type or paste <code>browser.tabs.hoverPreview.enabled</code> into it. That will then appear below with the word true next to it. Double-click true and it will change to false and the sidebar will be no more.