They’re really not so different, but you could think of Collections as temporary front-and-center tab organization around a subject. Tabs stay nested in a Collections folder for tidy storage and viewing on your home screen. Firefox Collections can be easily created when you have the need and then deleted when you no longer do. Bookmarks usually are never deleted.

You explain the intentions, but features are rarely used as intended by end users. They are used for whatever purpose the end users deems useful, based on what the design allows and promotes. What were the design principles that make collections useful even though you already have bookmarks in place?

Since collections are so much easier to access than bookmarks now, the natural tendency will be to use collections instead of bookmarks, grumbling a bit as you convert the bookmarks to collection entries little by little.

Hi

Bookmarks are unchanged - a pointer to a page to make it easier to go back to a site in the future. Bookmarks can be syncced with a different copy of Firefox

Collections are a group of tabs that you have had open that you may wish to revisit and can be grouped into a topic as required. This article may help: https://support.mozilla.org/en-US/kb/collections-firefox