i have anoher older macbook pro, but how an i have have the 'side panel,' with folders in my firebox browser like i have in my current macbook pro?

Hi, you can enable the sidebar in Firefox Settings (see this article), and you can use Sync to synchronize your bookmarks (see this article).

Make sure to update your old Firefox first, which is more than three years outdated — if you're looking for any feature parity. Your macOS Big Sur can run the most recent release just fine.