Long: After many years of many systems, I now use FF bookmarks on desktop as my base for managing and organizing. A few months ago I pared down my collection significantly but still have a few thousand links. I only sync across FF using the built-in sync. Used Xmarks previously for Chrome/FF/Safari but that got too complicated and broke once they shut the service down. If I need to sync elsewhere I can just export from FF.
I utilize the built-in "Bookmarks Toolbar" folder for work/frequently accessed pages. I have a "To Sort" subfolder in there for bookmarks that I still need to categorize. I make folders for things that I'm researching or articles I want to read on desktop.
In the main "Bookmarks Menu" I have about 25 folders for different subject areas like Business/Employment/Money, Design, Development, Travel, etc. I have one folder where I keep track of accounts I have logins to. I have a "Buy" folder with a few different subfolders of brands or stores that I like.
Finally, there's a "Mobile Bookmarks" folder that contains bookmarks saved on FF iOS. This makes it easy to save things quickly on iOS and organize later on desktop. If you use Safari on iOS you can easily bookmark in FF by clicking the share button, selecting FF iOS (must be installed) and choosing "Bookmark This Page". You don't even have to exit Safari :)
In general I bookmark because I like curating all these unique areas of my internet life. I like having a list of personally vetted links that I can go back to when I need them, after my brain has moved on and my browser history has been cleared.