I've used profiles for a long time, but they have some annoyances.

One thing I really want is a comprehensive system for transferring/syncing only certain data between profiles. Profiles contain some data that is specific to, well, a particular browsing profile (like open tabs), but also data that is really more specific to me as a person (like font preferences). And then each extension can have its own settings or data that I may or may not want to transfer. I always have to look up old articles on Mozilla wiki and hope they're still accurate when I want to transfer certain data/settings between profiles while neither nuking everything nor copying everything. It would be great to have a sort of "data browser" that let me pick and choose certain data and then create a new profile from that.

The other disappointing thing to me is how they talk about profiles (and container tabs) as related to different usage patterns like work, home, etc. I mean, yeah, that's cool, but what I really want (certainly from container tabs, sometimes from profiles as well) is site isolation. I don't just want one profile for my work Gmail and another for my personal Gmail. I want one container tab for "everything Google", isolated from the rest, to minimize Google's tracking. But, like I said above, I still want all my personal preferences uniform across these profiles/containers.