You can actually fairly easily unload tabs with about:unloads right now, but you have to do it in the order Facebook Firefox thinks they should be done for some reason.

Honestly, I don’t know why, but sidebar tabs have just never worked for me. It makes no sense, but for some reason my brain just doesn’t process them correctly.

But I agree, in general more fine-grained control of tabs would be the thing I would need in order to feel like Firefox was feature-complete.

Edit: Facebook? Wtf?