Have you looked into Firefox Multi Account Containers? I use this extension to quickly switch between multiple tab groups, some of which do use SSO while using the same service (e.g. Outlook). Plus, you can easily create new tabs like Ctrl + Shift + (#1-9) to open a container of your choice among other shortcuts. I know it doesn’t meet all your criteria, but each tab group has its own colour and/or icon to make them distinct, while being in the same window

Fun fact: the solar roof debuted on the Prius for the 2010 model year, in 2009

Perhaps right click on the sidebar -> disable compact mode? I haven’t had any issues moving tabs between windows, but then again I keep the sidebar persistent

Perhaps Ladybird once it’s released?

New New Mexico and Old New Mexico