Browsers suddenly keep syncing between two macs?

Hi, I'm not sure if this is a new feature with the Big Sur update (it has only started since then). I currently have two Macbook airs: a 2020 (running on Big Sur) and a 2013. Any time I open a Firefox tab on the 2013 (for example), it appears on the 2020 dock as if I had opened it on the 2020 and then minimised it. It's driving me absolutely insane, and occurs with Safari as well. If I hover my mouse over the icon, it only allows me to open it. But if I open and close the tab, it just does it over again. I'm not sure if I have to disconnect the 2013 from iCloud to make it stop, but I really would like to keep them both on iCloud so it's not such a hassle transferring files.

(Note: the same thing is occurring on the 2013 -- a tab I open on the 2020 will show up on the dock of the 2013, so I don't see how it can be a Big Sur problem given the 2013 is running on Mojave)

I've attached a photo of what I mean. There is no option to close the window without opening it. It will reappear after I close it. And I'm not sure if/how I can disable this feature.