I discovered the "multiple windows" trick/hack/bug to open 2 Zotero windows at the same time on a Mac, by minimizing the first window and then using Cmd+0 to "open" the main window which actually opens a second instance.

And it's been an absolute GODSEND -- I have a deep, tall folder library and often need to keep my space in multiple places at once -- so I can work in two far-apart folders at the same time, or keep one folder open while I search my library for a different document.

Now windows are great, but my laptop screen is only 13", so honestly what would be even more useful would be tabs. I can say I would normally use Zotero with 3-4 tabs open *always* -- one tab for my "main" folder I'm working in, a second and third tab for "related" folders, and a fourth tab kept open for looking up items in my library overall. And then be able to drag-and-drop between tabs by dragging to an open tab which would then, after a delay of a second or two, switch to that new tab -- just like Finder.

I don't know if the underlying Firefox engine makes it easy to expose native Firefox tabs? If not, it feels like it could be implemented by building a custom, relatively simple tab interface in HTML, and then embedded existing Zotero windows as iframes. That way you wouldn't have to worry about collisions of ID's etc. -- each iframe still operates entirely independently.