Bug 242207 was recently fixed on the Firefox branch, which means it'll make it into Firefox 2.0. This is great news, especially since I run into this problem all the time. The issue is that removing an entry from your address bar using Shift+Delete occasionally nukes your browser history. I can't tell you how many times I've run into this and then wondered what was causing my history to get lost. It's nice to see that someone has found the problem.