Experimental feature but can be enabled in firefox stable in Settings, Firefox Labs, Tab notes under Productivity.

I found it really useful for when Im researching something and have lots of papers open in separate tabs. Though Im using it more like tab tags, since notes show up when you hover over tabs. It lets me see at a glance which specific topic a tab is related to if I forget.

What do you all think? I was kinda surprised how little discussion I have seen on this, considering that no other browser have something like it. Firefoxs implementation is pretty smart too, instead of just being a sticky notes widget in the browser (like the one in Vivaldi).