Sync tabs between Tor Browser instances
We can think about synchronizing (some) open tabs between Tor Browser instances by each instance creating and registering onion services with other instances. The first iteration of this doesn't need to be crazy where we synchronize the exact viewport location of each page between instances, and it shouldn't synchronize all open tabs by default.
But, it'd be neat if we could select a tab in the browser and choose to which other Tor Browser instance that URL should be sent. The URL should not be opened automatically in the other instance, but it should be listed "somewhere" in the UI. If one wants to open one of the synced tabs, then they can open the list and select it.
To some degree this has the ricochet problem but it shouldn't be nearly as bad. The worse tradeoff is that both Tor Browser instances must be online at the same time. This could work well enough for desktop->mobile, but it'll be harder in some situations for desktop->desktop, and mobile->desktop.