Great project! How alive is it?

If the add-on does not work and shows blank Aggregator/Archive in preferences, edit and enter this url:
https(colon)//timetravel(dot)mementoweb(dot)org/timegate/

I found the link at the bottom of official page:
https(colon)//mementoweb(dot)org/depot/
I also contacted developers in hopes of getting a fix.

Works OK, but only on desktop, not mobile. It's nice to have this feature in the browser, instead of using mementoweb.org. It's even better when the target web site supports the memento protocol (RFC 7089). This add-on's user interface is a little clumsy, with choice of desired date/time in one place, and choice of actions to perform in a different place. (Actions are in a context menu, reached by control-click on a link or in the page, and selection of date/time is in a different menu behind an icon in the top bar of the browser.)

Doesn't work. All it does is add the word "undefined" before the URL, which obviously doesn't achieve anything.