I'll be the first to admit that blogging about not-so-obvious features in software isn't always cool, unless that happens to be your thing. However, I somehow missed this (almost) amazing functionality in Firefox until just now.
When you highlight something in a page, right-click that selection and click 'View Selection Source'. Voila, you have the ability to view the modified DOM source!
Ideally Firefox should have another button for the DOM source without you having to highlight everything, but this does give you the ability to highlight only the portion of the page you're interested in. Also, I don't know why this isn't an option in the otherwise amazing Web Developer toolbar.
Thanks to Jeremy Fujimoto-Johnson for pointing this out in a comment on the above-linked NCZOnline site.
Update 2006-01-05: Hooray! The just-updated version of the Web Developer toolbar greatly improved all menus including a new "View Generated Source" function.
I never knew that. Now it will be 100 times easier to write my AJAX apps. Thanks!
You may be interested in the fairly new extension FireBug. It has nice DOM browsing functionality and shows a decent view of the DOM source.