Since several years now, Firefox choose to make "Add Bookmark" window (Ctrl-D) a fixed size. I really like the workflow for the user of clicking on the url bar's star then choose a location in my bookmarks folder to save it.
That's the point where the user experience become cumbersome and, after a while, really infuriating especially on desktop.
To fix those issues, in "Add Bookmark" menu (Ctrl-D):
On a side note, I did research how to resize this window. I found old posts and issues spanning several years and many subpar solutions about extensions (most doesn't exist anymore) or userChrome.css file to bypass this UI problem.
I hope it doesn't come to harshly on the Devs of Firefox. It's an amazing piece of software that I love.