From the “For Web Developers” page linked to from the new Mozilla 1.0 start page, I found the Mozilla source generator sidebar, which generates formatted HTML source based on the current page’s DOM. Interesting idea.

One complaint: the output is HTML, not XHTML, even on XHTML pages: the tags are upper-case. This does most decidedly not mean that this is not a useful tool. I think it just needs a bit of tweaking.

Take note of the instructions to add this line to your prefs.js:

user_pref("signed.applets.codebase_principal_support", true);

The script that runs in the sidebar needs elevated permissions, which this preference setting allows.

Note, too, among the documents, a Guide to Mozilla 1.0, which brings together links to some very useful information.