If you haven’t seen Firebug then you’re definately missing out. Firebug has been out for quite a while, it’s nothing new; but I think it never got the attention (especially in .NET developer circles) it deserved.
Here are some nice screenshots I just made of how I use Firefox:
Screenshot of Resources Loading Clientside
You can list your resources that are loading (images, external files, etc.). By doing so, you can see load times as well as broken links or clientside errors (marked in red) while loading.