It's configurable from the sidebar dropdown menu, e.g. when opening your bookmarks (Ctrl+B):

But you can also use this about:config setting:

Again you keep calling it a popup, but it really isn't. It does not pop up or overlay the browser viewport, it sits on the right and pushes the viewport left reducing its width.

I've donated in November after I switched back to Firefox as my main browser. I read about the search deal dependency and wanted to contribute to what Mozilla called "reclaim the internet". Feel something akin to 'buyer's remorse' when I now read how little goes to development of Firefox/Gecko (the only multi-platform alternative engine for rendering the internet) and how much goes into CEO salaries.

No, there are 'workspaces' -accessed via the buttons on the bottom of the vertical tab bar-with their own tabs, but you can't group tabs within one workspace (yet). And yes I can open the sidebar -which is not a popup- on the other side. The thing that's popping up on the left next to the tabs and overlays the site is what Zen calls the web panel.

That popup you mention sounds like the zen web panel rather than the sidebar. In total that gives 3 concepts: vertical tabs, a floating web panel next to that and the sidebar. I like what they're trying to do, but it's not all polished yet with some odd UI glitches or things requiring (re)configuring like the toolbar. That combined with bad font rendering and missing tab groups made me switch back to FF for now...where I now disabled all usage data sharing options.

Indeed. It uses some of the vertical tabs features in Firefox, but adds workspaces (~tabbed tab window) that you can assign default containers (like banking, personal, shopping) to. Tab groups are missing, but are being worked on. Don't expect nested tabs or trees like Sideberry or TST offer though. Honest disclosure: the implementation is not fully polished yet. The location where new tabs are opened or their remembered location after closing/opening the browser is not consistent.

Yes, I have tabs on the left and sidebar opens on the right.

Upvoted it. The inability to use a sidebar (local LLM chat) on the other side with vertical tabs or at all when using sideberry together with their changed stance on selling user data made me move to Zen browser.