2022-10-27 update:
Doesn't seem to be working as reliably on new tabs on Firefox 106 as it used to - but I can work around it by toggling the icon off and on in the toolbar before clicking each link, which is still easier than dealing with a bunch of separate windows.

One may be able to reproduce the issue by dragging a tab to another window (which seems to cause the single window add-on to lose track of it), then try clicking a pop-up link and observe that the link opens in a new window. E.g. on webbrowsertools dot com's popup-blocker test.

2021-09-29 original review:
Invaluable when working with Fieldpoint, which wants to either show everything in multiple columns in one window (impossible to handle multiple tickets), or always pops out to a separate window when "pop out" is enabled (no built-in way to work with tabs in Fieldpoint). This Single Window extension forces the pop-outs to open in new tabs instead, and makes working with multiple tickets much more practical!

My only desire is if I could set it to only enable on certain domains. It doesn't cause much trouble, but I do have to remember to toggle it off or it interferes with the print button on eBay invoices.