I think this is related to invalid character problem reported earlier, but I found that sites with ':' in title will not work. For example, using Bookit on the following site will create empty file names 'Ctan'.
http://www.ctan.org/what_is_tex.html
If you can allow Bookit to automatically replace or remove such characters, it will be really helpful.
Thanks for your great extension and it works great in FF3 without any problem.