Bookmarks Manager on Macbook Firefox

I am constantly building a default tabset of approx 10-15 tabs in my FF browser on my Macbook (in case something differs from FF on Windows. I have a default tabset saved in my bookmarks. But when I add a new tab or rearrange their order and I go to save all tabs selected, is there a way to just over-write my current bookmark tabset? All I have been able to do is keep adding and "nesting" in my bookmarks which makes a mess - a dogs breakfast to the bookmarks manager set. Even when I select my bookmark default by name, it simply keeps the original and keeps "nesting" under it, which takes more time to babysit in cleaning it up. Wish FF would spend more time on bookmarks and clean this up so I can simply "overwrite" rather than nesting and building a "tree of bookmarks"?