Type about:config<enter> in the address bar. If a warning screen comes up, press the I Accept the Risk button. At the top of the screen is a search bar.

browser.tabs.selectOwnerOnClose = true

Thanks for the quick answer, but it's not really what I want. Let me give a little more detail. Suppose I have three tabs open. I'm reading tab two and I open a fourth tab. I go back to tab one to read something, then back to tab four. I close tab four because I'm done with it. What I want is to go back to tab one. That's what I mean by "the last active tab"...the last one I was reading. This fix doesn't do that.

With your fix, closing tab four seems to take me back to tab two, which was active when I opened four.

That's all Firefox has to offer. You might find an add-on that can help.

rx2v2x said

What I want is to go back to tab one. That's what I mean by "the last active tab"...the last one I was reading. With your fix, closing tab four seems to take me back to tab two, which was active when I opened four.

Try using this extension. Haven't used it in Firefox 57 just yet -- in fact I just discovered that it appears to be compatible with Firefox 57. I'm a late adopter and avoid new major updates for a few versions. Focus Last Selected Tab Switch to last selected tab when closing tab. https://addons.mozilla.org/en-US/firefox/addon/focuslastselectedtab/

Thanks to both of you. I also left Firefox for a while. I've been using Vivaldi instead, but thought I'd come back and try the new version. As you can see from the attached image, this is a no-brainer in Vivaldi. Not sure it's worth fooling around with an extension, but if I get a chance, I'll give it a shot.