If you were using Firefox 1.0 - 1.5, you might have several hundred bookmarks files. Check within your user files under yourusername/Library/Application Support/Firefox and you should see bookmarks.html but may see bookmarks001.html or some such. Deleting them will also free a lot of space, depending on how much you browse.
At one point after installing 1.5.0.7, I ended up deleting my whole Firefox directory to clean up things, having saved the bookmarks file. It stabilised things quite a bit, though I had to re-enter data and settings.