Please update add-on to latest version (v2.2)
In this version I fix duplicate groups/tabs.

About "about:config" and etc.: this is not supported bu Firefox WebExtentions API
https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/tabs/create

For security reasons, in Firefox, this may not be a privileged URL. So passing any of the following URLs will fail:

chrome: URLs
javascript: URLs
data: URLs
file: URLs (i.e., files on the filesystem. However, to use a file packaged inside the extension, see below)
privileged about: URLs (for example, about:config, about:addons, about:debugging). Non-privileged URLs (e.g., about:blank) are allowed.
The New Tab page (about:newtab) can be opened if no value for URL is provided.


Version 2.1 and above has option to fast group switching (in options page)
Thanks for your feedback!