Simplified Tab Groups is a brand new add-on for the Firefox web browser that aims to provide Firefox Panorama / Tab Groups users with a replacement for the native implementation that Mozilla plans to remove in a future version of Firefox (likely Firefox 45).
The reason for removing the feature, which was part of the Firefox web browser since version 4, is a lack of interest from the Firefox community and high maintenance costs. In other words, too few users use it to justify the costs of keeping the feature in Firefox.
Obviously, part of the low number of users who use it, Mozilla stated a while ago that 0,01% used it, comes from the fact that it is not promoted at all by Mozilla. This is actually true as well for complete themes, something that Mozilla plans to remove as well from Firefox.
Firefox users who use Tab Groups in the browser are left with little options. Accept Mozilla's migration plan and get all sites open in tab groups saved as bookmarks, don't update Firefox to keep using the feature, or hope that someone creates an add-on to continue using the feature in Firefox 45 and newer.
Tab Groups, a new Firefox add-on could be that solution.
Note: The add-on is fairly unstable right now. It sometimes displays blank information instead of the groups and tabs that you have created, and at other times loses a whole batch of sites open in a group. According to the developer, this is caused by Panorama still being available in Firefox and will stop being a problem when the feature is removed from the browser. The author notes furthermore that users should only install it in Firefox 45 or newer.
Good news is that it supports native Tab Groups data which means that you may use it to retain your groups and the sites in those groups.
The add-on displays all existing groups and sites when you click on its icon. You may use the interface to switch between existing groups, switch to sites directly, create new groups, remove sites, or remove groups from the interface.
The interface is basic compared to Firefox's Tab Groups feature as it does not show sites and groups visually on a page. There is also no option to move sites from one group to another (yet).
Tab Groups is the first replacement of the same-named Firefox feature. While it is necessary to get used to a new UI, it is the first add-on that offers a glimmer of hope for Firefox users who use the feature in the browser currently.