Hello, I have around 2500 bookmarks, and some users have 80000 bookmarks, and it is working fine. It must be something else like maybe a bug or something special in the bookmark DB which is making Firefox not answer the add-on.
I will need more details then to pinpoint the problem. Can you open an issue on https://github.com/aaFn/Bookmark-search-plus-2/issues ? Thank you, aaFn