Sadly this is a limitation with Firefox's WebExtensions API. On the off chance that this was an oversight on the Firefox team's part, I've submitted a bug report in hopes that this can be resolved. Unfortunately, until then I don't believe this extension, or any other extension which closes multiple tabs, can take advantage of the new "Undo Close Tabs" feature.
If you are aware of any extensions that do work as expected with the "Undo Close Tabs" button, I'd be eager to take a look so that I can get this fixed!
