Tab groups and sync extensions has already been suggested but yet to be implemented.
With Firefox it is possible to send (copies of) Tabs to other device as well as open up (a new copy) of tabs from other devices with the Synced Tabs feature. Using these are tedious and gets arduous. Plus now you have the same website(S) open on different devices and will show up in the "Synced Tabs" as separate tabs making the Sync Tabs list bulky and inefficiently redundant.
I work in one window, multiple tabs, (its easy to loose tabs when working in multiple windows).
I want when I close a tab on one device, it closes on all my devices. Open, change URL, etc. While this may not be good for classic browsing on multiple devices, using separate spaces/groups that sync together turns multiple devices into a single seamless continuity.
tab groups are organized by user preference and not device and by Syncing tab groups it will feel like working on a single device instead of multiple devices. Closing a tab in a group will mean that the tab is closed on all devices, etc. Separate devices can have different active groups.
Extensions exist that can group tabs, backup, and restore, but they cannot be synced live which is something I believe Firefox can and should implement themselves if they are not willing to sync all extension settings/data.
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This would be perfect for me. I frequently switch between working on my desktop and laptop and making sure all the tabs are synced is a pain. It would be even better if the data from add-ons is also automatically synced as I make heavy use of tab groups.