I began with del.icio.us by casuality: I was reading a post and was really tired of seeing that name all over the places so I opened an account and then I realised how important was it. Then, I found the Yahoo my Web 2 and I thought it was very nice, nicer than del.icio.us.
The Toolbar works perfectly with firefox and has many features that makes it really useful! (it looks like I'm sellings it 😛 )
Have you tried it yet? 😉