It seems to be normal behaviour. You would have to close the sidebar before opening a new window.

Thank you for responding. That used to be the case… until recently. Now it opens all the time, even when all bookmarks are closed. New windows & tabs... It started happening after the last FF update. Hopefully there will be a solution, it’s time consuming with my research workload.

Will appreciate any information/help you can provide.

Thank you again for reaching out. Jemmma

You are welcome. Sorry to hear what is happening. There is a thread from 2018 whose solution may help. https://support.mozilla.org/en-US/questions/1228586