How can I overwrite old tabs when opening new ones in Firefox 57?
Before Firefox 57 I used an extension called ReplaceTabs that would overwrite existing tabs when I opened new ones. This extension doesn't work n Firefox 57 and I couldn't find a way to enable this functionality.
The best I can suggest to most quickly dump the old tabs is to Shift+click the Open All in Tabs button to launch those tabs in a new window, then manually close the old window.
But maybe someone is working on a new add-on for this.