I know there's been a lot of discussion about Firefox vs. IE but I found one more thing that I like about Firefox today. Maybe other web developers know this handy step already so this is for "newbies" like me - don't flame what I'm about to say.
When validating a site that is PHP/MySQL, I found an easy to way to fix HTML code errors in Firefox. Using SiteReportCard, I was reviewing my HTML Code and found a few warnings; the warnings returned the line # with the code error. Since the site is generated by several includes, my line # did not correspond with my template in Dreamweaver.
The solution? I opened the site I was checking in Firefox, clicked on View | Page Source, which opened my code in Firefox (not Notepad). I then clicked on Edit | Go to Line... and entered my line number into the field. This showed me where the error code was generated so I could then refer to it in Dreamweaver and fix the error.
I know this made my day a little easier!