So: it'll eventually land in Firefox, see https://wiki.mozilla.org/WebExtensions/TabHiding -- in order to be close to tabgroups, we would need Milestone 2 to be completed.
With FirefoxNightly and some fiddling, tab hiding works already (see https://github.com/kesselborn/conex#getting-a-version-that-can-really-hide-tabs ) ... however: you only want to do this if you are open to experiments ;)