This would be a huge improvement - Chrome does this on mobile. I can save a bookmark to a specific folder in my bookmarks toolbar, then the next bookmark I save will default to that folder. 1-click bookmarking that ensures I can then find that bookmark when I return to my PC. I often do this when I find something interesting I just want to look up on my PC later.
Firefox defaults each new bookmark to 'Bookmarks', the level above the Bookmarks Toolbar, so I have to do the following each time:
also worth noting, if I do those 6 steps, and then sometime shortly after I bookmark a new site, it will respect the previous folder I had selected. There is some sort of timeout where it will again default to 'Bookmarks'