bsmedberg, I need help to implement this correctly. Can we talk today or next week on IRC?
In general it's easier to ask "what % of users active in a given day/week/month used a container tab." That is not a hard thing to measure using the longitudinal dataset. Unfortunately there are basically no docs on how to use this, so I'm going to forward you to RyanVM and he or his team can help.
Andrea, please contact me on IRC.
Good news: we don't need any extra telemetry IDs. We can do our analysis using a.t.m.o (aka telemetry analysis service). I wrote an python script to retrieve this information. Onces I have some result, I share them and the script as well. Just because there is nothing to do from a gecko point of view, I close this bug.