Is there a firefox plugin that allows you to edit the source code of a web page?

I am looking for something that will allow me to edit the source of a page and then have it updated in the browser window. If the page links to 'image.gif' then this link should still lead to image.gif in the directory that the original html page is from, not some directory on the local machine.

Answers:
You can do this with the web developer extension. Just go to Miscellaneous and then Edit HTML on the toolbar.