Better than Mozex Rated 4 out of 5 stars
ViewSourceWith Works as advertised. I have been a big fan of Mozex, but it's buggy in that the source it sends is different from the source Firefox receives (for example view the source of non-well-formed XHTML). ViewSourceWith, by contrast, correctly presents the original source.
Also ViewSourceWith's "Use in place of native view source" is a great feature (although it seems to fail on pages with post data).
One of the other problems with ViewSourceWith is that many of the features are poorly described, as its author clearly is not a native English speaker. There's no way to be sure how much of the extension I'm missing. But the parts that I do understand work quite well for the most part.
I'd rather use Mozex for view-source, since I need it for textareas and mailto anyway, but until it gets fixed I'll be happy to stay with ViewSourceWith. It's quite useful. I feed my source to Vim, which then automatically parses it through Tidy.