It would be literally impossible for me to do my job effectively without Sidebery for container management in FF. As someone with multiple accounts used for various CSPs and other services, being able to spin up an environment with no irrelevant cached user sessions is something I depend on heavily (looking at you, Azure).
I can't speak for Sidebery, but Firefox's own Multi-Account Containers extension indispensable. Give it a try (maybe without Sidebery enabled in case it causes conflicts) and you'll notice how much it helps with managing multiple Azure accounts and other general browsing isolation. Particularly useful is the "always open this website in this container" feature.
There was a reason I switched to Sidebery over The native implementation but I can't remember. Maybe it was the Panels feature that FF Containers didn't offer at the time.
I do miss some of the functionality of the Mozilla add-on though, I don't think they opened up APIs to some of the extended tab/window options.
I love sidebery, but can’t use it anymore due to the amount of slow down and stutters I’ve experienced with it. If my browser is open for more than 4h or so I starts taking up to 1-2 seconds for me to create a new tab or switch between tabs in it.