Does support email work?

You can check your email and open the URL that was originally emailed to you. Mine still works as of today, after almost a decade. You might have to restart your browser afterward for it to take effect, and you'll have to sign into Chrome from the Tabs Outliner settings page and allow permissions and all that stuff again

Thank you, Aidan!

Clicking on activation link in the email from TabsOutliner helped.