The best new feature of Firefox 1.0 is the ability to open links from another application such as email and not have it open on top of your currently opened web pages.
From the file menu:
Tools > Options > Advanced > Tabbed Browsing > Open links from other applications in > a new tab in the most recent window
I've been using Firefox as my full time browser but had left Internet Explorer as the default to prevent a clicked link opening over an existing page. No longer the case.