One of my favorite bits of software is Xpra [0], "screen for X". You'd run it and it would start another X server (start apps in it with `DISPLAY=:1 xterm` or whatever), and you would "attach" it to your running X server with `xpra attach`.

You can attach to e.g. `ssh://hostname/:1`, so I ran a firefox instance on a homelab server and attached to it from my laptop and my desktop to not have to bother keeping bookmarks, tabs etc in sync.

[0] https://xpra.org/