Ctrl+click the link (Mac: Command+Click) 

middle-click the link that you want to open ie: go to the page (e.g., mouse scroll wheel) Ctrl+click the link (Mac: Command+Click) Should put it back Alternatively there are Extensions that do this https://addons.mozilla.org/en-US/firefox/addon/open-link-in-new-tab/ Also should check for malware to see that nothing malicious changed it https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware Please let us know if this solved your issue or if need further assistance.

Actually found that a malware had added "packageho.me" to the addons. Went to Extensions and removed