THIS! I came to suggest the same thing...and was happy to see that someone had already suggested it and it appears that nearly 100% of the comments are in support.
I live with firefox - eat, drink and sleep with it. I have screenshots of warnings I was about to close thousands of tabs on closing it. Of course my options are set to reopen the same set of tabs on restart 😛
Needless to say, I struggle with organization of all the loose ends I am researching. Groups are an amazing help, but pinning them even more so.
With tabs pinned to groups and groups pinned to the toolbar, I could safely wipe my tabs now and then without affecting those I rely on for my continuing day to day activities.
Please, pretty please, escalate this feature.
I have used firefox since it was netscape and have avoided M$ IE and edge ever since little billy was chastised for using OS releases to stunt and bury independent apps like wordperfect, netscape, dBase etc.