Remember that Firefox's sandbox is still substantially weaker (especially on Linux, where it can hardly be considered a sandbox at all) then Chromium based browser and lacks support for isolating sites from each other rather than only containing content as a whole.
So if one exploit Firefox itself, then all of your content would be not secure as it's not isolated.
That's why Chromium based browser have (strict) site isolation which also protect against sidechannel attacks too.
With all due respect:
1) FPI (First Party Isolation) + Containers = both privacy features... have nothing to do with sandboxing.
2) Sanboxing feature is related to SECURITY, and this post is about PRIVACY. More specifically, this post is about one add-on/extension and privacy... that's all.
3) I have nothing against Chrome/Chromium. I just followed the logic. And considering that this post was focused on privacy, then all my explanations were related to Firefox. I don't want to sound rude, but it's a contradiction to worry about privacy and to use Chrome/Chromium at the same time.
If you want to completely change subject, and you want to compare Firefox' security with Chrome/Chromium' security, be my guest, open a new post and talk there, for example, about sandboxing.