When I use Control-B to bring up the Bookmarks sidebar, it takes maybe 2-3 seconds to appear.
Yet, if I click on the "Bookmarks" Menu title at the top of the Firefox window, it freezes Firefox for about 20 seconds while it works to open the drop-down window.
What's the deal there? I do have a lot of Bookmarks saved over the years; I'm just thinking the two routines load the file differently.
I don't have an answer, but the fact you say you have a lot of bookmarks leads me to ask if you do or how you back your stuff up. If you don't have this, get it, since it's free:
http://mozbackup.jasnapaka.com/
It works with FF and the Mozilla suite and T-bird.
One of my biggest gripes is how FF has had a bookmark problem for who knows how long, and they've yet to address it.