Is it possible to view HTML source auto-formatted in Firefox

I am writing a Web site in PHP, and the HTML it renders is not well formatted and hard to read in Firefox (most of it is on a single line).

I don't want to mess up my PHP code by adding loads of white space formatting stuff, so I was wondering if there is an option or perhaps an add-on to Firefox that will give me the ability to view the source indented according to the HTML's structure.

Solution 1:

View Source Chart 2.7

[UPDATE]: original link was broken, here is the website: http://viewsourcechart.com/

Right click -> View source chart

This is what I get for the Google home page HTML source:

Before:

After:

Solution 2:

From here

View Source Chart Graphically displays source structure.

JSView 2.0.5 Displays information on page external files.

HighlightAll Highlights identical elements in the code.

Html Validator Validates HTML of the current page.

View Formatted Source Shows source structure.

SourceEditor Allows to edit code and view the results.