This seems to be dead simple, you export your bookmarks to html or (html or json file in case of Firefox) and import it back to another browser. Problem is that browsers export all bookmarks and import also all, you can't select just one bookmarks folder to export. So unless that is fine with you, you need to edit html file to prune the bookmarks, or just live with them all... There is also a workaround - you select & copy the bookmarks you want to transfer in the browser's bookmarks manager and paste them into some suitable program, I found Notepad++ works and can save out html files. Over the years, I kept exported bookmarks html files as backups in various places and when I discovered Brave browser this spring, I imported them all into it. Recently I found some shortcomings of that browser and decided to try Opera and settled on the portable Opera. But didn't want to import all the bookmarks from Brave browser. I found quite handy free program StorURL here https://www.freewarefiles.com/StorURL_program_13313.html and imported html bookmarks file into it which I previously exported from Brave browser. Its advantage over Notepad++ is that it is made for the purpose, so it keeps the bookmarks in their respective folders. You can export a subset of them to html file which you then import into your current browser. Bonus is,during installation you can choose to install the StorURL as portable and keep all your old bookmarks stored in it on some backup place, ready to open and search it or just browse it for nostalgia reason. Just to give you an idea of my situation, here is just a small part of my collection visible. Typically, each dated folder contains some 20 to 40 or even more bookmarks. Here you see just a small part of it. BTW the Greene Murder Case is nice movie, the book it is based on is even better. I like them all as well as the movies. The author's (pseudonym) name is S.S. Van Dine.
