Most Recent Tab is about quickly toggling back to the last tab you were on with a keyboard shortcut or toolbar button click.
The default shortcut is set to Control-shift-1 on Windows and Command-shift-1 on Mac.
Version 1.0 is a complete rewrite in the new WebExtensions API to support current and future versions of Firefox. Unfortunately it's not currently possible to customize keyboard shortcuts with this new API, but the Firefox team has a bug (#1303384) tracking this feature. Stay tuned.
Source code is now available on Github, under the Mozilla Public License 2.0.