The HTML Tidy plugin for Firefox is a great little tool when viewing other webpages’ source. It shows any errors, where they are, and what they are. It also has a clean-up function for those pesky unreadable CMS made html codes, making it easier for the human eyes and brain to read.
If you’re on Windows you’ll get HTML Validator off the Firefox add-ons page. For some reason Mac-users must do a slight detour, which I found after some googling. Go to the HTML validator sourceforge index page, and find the file called tidy_macintel_xxxx.xpi on the date of 26-Oct-2006. That’s spesificly for the new Intel-Macs, but you’ll find any other version you’re interested in on the same page.
Update: The Mac-file I’m referring too is for Intel Macintosh’s only! The filenames on the sourceforge page gives you a fairly straight forward clue to which OS and make it’s for, so remember to read before installing.
If you try to install the macintel on a G5 or earlier Mac it will cause Firefox to completely give up on you. The fix is then to export your bookmarks, delete Firefox, re-install. And then start adding all the extensions you’ve lost in the process.