Usually, the back button will be greyed out if there's nothing to go back to in that tab. So if you open a site in the first tab, but then open subsequent links in new tabs, then you'll be able to go back to the previously visited pages by right clicking the back button and selecting from the list in those tabs. However, the back button in the first tab will remain greyed out. Install this add-on if you want the little arrow on the back/forward buttons again: https://addons.mozilla.org/en-US/firefox/addon/backforward-dropmarker/ If you did go to more sites in a tab, but the back button still remains greyed out, restore the default set, instructions here: [https://support.mozilla.com/en-US/kb/Back%20and%20forward%20or%20other%20toolbar%20items%20are%20missing#w_restoring-the-default-toolbar-set Restore Default Set]