(On Windows anyway, don't know if different on Linux)
Just wanted to share that as a user of both Firefox and Chrome, it's one thing that makes me hate switching to Firefox. I often need to use two different profiles and the way Firefox does it sucks.
With Chrome I've got two shortcuts (that Chrome creates by activating an option) pinned to my taskbar that look distinct from one another and the instances that I open are combined under their respective profile shortcuts.
With Firefox I need to manually create two shortcuts, assign two distinct icons to differentiate them, change some properties so they open the right profile, pin them and because they're "regular shortcuts" instead of the default Firefox launcher shortcut, when I open the program I end up with a third Firefox icon in my taskbar (it does not open under the shortcut I used, it acts as if I clicked a shortcut on my desktop) where all instances get merged together no matter which profile they're associated with.
Then use the Multi Accounts Containers extension, which achieves exactly that. I have work under the default container and my personal accounts in a "personal" container. We both use Google accounts, and it's simple to separate them.
Link: https://addons.mozilla.org/firefox/addon/multi-account-containers/
It's an official extension by Mozilla. And I can confirm that it's amazing, and if multiple website accounts are your use case, it's better than profiles.