The killer feature of containers for me is keeping Google isolated. I've degoogled my personal life but still have to use it for work (i.e. G-Suite SSO). I can force all those sites to open in my work container which leaves my other containers Google-free.

Before Firefox I used Brave with multiple profiles and it was very painful to use in practice; two sets of extensions, two unlocks of 1Password X, no ability to force certain websites to open in Profile X or Y, etc.

But yes, Firefox feels like it's getting progressively slower and I'm finding more and more things that don't support it. In particular, many of the recent spatial audio tools just flat out refuse to run in Firefox, and tools like Apple Business Manager don't run even though they typically work fine if you spoof the User Agent.

If a Chromium-based browser like Vivaldi or Brave added containers it'd be a very compelling alternative.