Have you tried display.clearViewer() between instances? Additionally, its a known bug if you try and plot multiple things the mask will only be from the last image plotted. I don’t believe that has been sorted yet.
Edit: I do realize this doesn’t fully restart the firefly instance but it may help.
To be clear, you do not have your own firefly “server”. There are one or more firefly servers (depending on the deployment) serving all users . (Brianna 's answer is correct, I just wanted to avoid future reader confusion.
For future interest, what is the language one should use to refer to the thing that is running the display that shows up in the Firefly tab. “Session”, “Instance”, “View”, “Window”?