How Do I Launch Firefox with Specific Pages but Prevent Said Pages from Loading in Subsequent Windows?

So, it's "migrate to Firefox because Google is doing more evil things to the web" day. Because of my need to remain constantly connected to my email, I configured Google Chrome to launch several pages at start up: my personal email, calendar, and drive, my university email, calendar, and drive, Facebook Messenger, and Messages by Google. I've found a way to configure Firefox to load these pages at browser start up, however this option also loads this set of pages every time I open a new FF window, which is sub-optimal as I like to keep the first window free of any other junk while using subsequent windows for browsing. Might anyone have any suggestions as to how I might get the desired functionality (launch Firefox and get a window with the above tabs, but not have them re-spawn every time I open future FF windows)? Thanks.