I combine containers with running multiple copies of Firefox under different profiles. I've been using multiple profiles for years, and I think it still offers some advantages even with container support. The big one is that one instance of the browser can crash or be restarted without any impact to the others. It also distributes resource consumption over multiple independent processes, though I'm sure there's some overhead in this approach. Finally, it allows slightly different settings for each profile depending on need. I manage common settings with a user.js file.

One profile is for logging into various services I regularly use, one is dedicated to day-to-day surfing, one is for local development (there are no addons in this one), one is specifically for webmail, and one is specifically for Trello. Containers are most useful with the first two profiles...especially the first, so that the various services have some level of isolation.

EDIT: details, typos, etc