Story 1: I am one of the legion of people with dozens of tabs open on device A that I would love to read or otherwise process (read it, fill in the form, finally buy the shirt ...). They aren't worth bookmarking but I have to get to them someday. Later, while device B is in front of me, I have some time to process tabs. Using Firefox sync, I open a few of the tabs on A and do what I need to do. I am now done with these tabs, they no longer spark joy, and I can finally remove some clutter in the list of synced tabs and on device A.
Story 2: Device A is a work computer where some tabs are useful only at work, but a dozen of them are shopping tabs I was browsing while on company time. I have time to finally pick a shirt and buy it at home, but the clutter of work-related tabs makes it hard to find the shopping tabs. I never want to see the work tabs on the computer at home--they're more clutter and some I can't even access outside of the work network--but I don't want to turn off sync entirely.
Solution: First, allow me to remove items from the list of tabs hosted on device A displayed in the list of tabs on device B. For example, right-click on the tab in the list of synced tabs and select 'hide tab on this device'.
Second, just as we can push an "open tab" request from one device to another, use the proposed right-click menu to push a "close tab" request. When I get back to device A at work tomorrow, I can accept the close tab requests, one at a time or all at once.
Alternatively, I have an "open tab" option, but also a "steal tab" option. The steal (or transfer) option simultaneously opens that tab on device B and closes it on device A (or if that is technically not possible, sends a close tab request).