Yes, that's quite practical. Together with the Web Developer Toolbar, Firebird is already a really great tool.
Addendum: The Toolbar used to be good - version 0.2 ran well on Firebird under OS X, version 0.3 doesn't work at all. And the author has such wonderful HTML on his pages that you can't find an email address anywhere and the contact form is of course broken (validation with JavaScript code that won't accept my email address). Idiotic communication prevention. Another addendum: ok, that was only with Safari and Firebird 0.6.1, not with Firebird 0.6 - at least sending an email works with that. But the toolbar still doesn't work...
At Markus Kniebes Journal you can find the original article.