The engagement on that experiment wasn't very high, but I later went and read some of the user research and felt like there was some unmet potential: the concept was understandable, the functionality matched at least some real use cases, but it still didn't work, people didn't use it.
Obviously Pocket is intended for this purpose as well. It's personally what I use for this purpose, though there's always a kind of in-between state where you aren't quite sure what you want to do with a tab. If you make it easier to save the tab, there will always still be a point when you aren't sure if you want to save it or not...
As far as I'm aware Mozilla hasn't done deep-dive research into tabs. Not all of our research is public, but we do post a public index of the research: https://firefoxur.github.io/