[idm]

I recommend the Multi-Account Containers extension by Mozilla, which has a "sort tabs" button that accomplishes what you want.

https://addons.mozilla.org/en-US/firefox/addon/multi-account...

I am not sure why this functionality is a separate extension - this is a bit confusing - but it's working for me.

[Timpy]

I'm using Multi-Account Containers, I never paid attention to the Sort Tabs button. This is great! I'm looking for a keyboard shortcut for it now.