Other good FFox uses:
Tidy - (x)html compliancy, (x)html validity.
Color Picker - Get the hex value of any pixel in your browser from the current page.
YSlow - See why the page takes so long to load.
Browser Timer - catalog how much time you spend both online in general and at each site.
No Squint - set font sizes on a site by site basis, so hard to read sites are no more.
webmaster toolbar - allows tweaking of values on some sites (including hidden input variables which can be fun)
XRay - see in-line the html values of a page as they occour

Hundreds/thousands of useful addons (and millions of crappy ones)

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