So I figure this should be a quick fix but I don't know where it is.
I want to know how to keep my original tabs in place when opening a
group of tabs (a folder of bookmarks) from the bookmark menu. I want
them to open up in order next to my original tabs. Currently, all my
the current tabs get removed and only the tabs from the bookmark menu
show up.

I have the Tab Mix Plus extension, if that helps.

Let me know.