HTML source appearing different in Firebug to standard browser 'View Source' option?

I have some HTML content being generated via some PHP.

Whilst investigating a css problem, I noticed through Firebug that some elements in the DOM were not organised as I expected. Yet, when I did the standard 'View Source' in Firefox it showed everything to be correct.

I know the source being displayed by Firebug is accurate, because the source it presents me corresponds to the aesthetic issue I'm seeing on screen, but I'm not sure what this means and how to investigate further.

Why does this happen, and which source version should I be looking at? (p.s.开发者_Python百科 I have no JavaScript running on the website.)

Firebug cleans up the DOM tree, so if there's any syntax bugs in the raw source, you won't see them in Firebug (unless they're so bad it screws up the parse tree completely).

The regular view-source functionality shows the page's source as it came from the server. If you do any manipulations of the DOM after the page loaded, it won't show up in view-source, as that's now outdated. Firebug will show the live in-memory tree, with any manipulations included, but it will also clean things up.

Firebug shows a live view of the page's DOM structure.
View Source shows the original HTML received from the server.

If you modify the DOM using Javascript, the changes will only appear in Firebug.
If your HTML was invalid and the browser fixed it up, the fixes will also only appear in Firebug.

You can use the browser's View Selection Source option to show the source for the actual DOM, which will match what you see in Firebug.

Firebug shows more than just the code you have entered. It also includes default styles from the browser (assuming you have not used yahoo css reset). Although you cannot guarantee that firebug in itself does not contain any bugs, I tend to trust it more the view-source, even more so when javascript is used, because the output of the page can be vastly different from the original html content, albeit not in your case as you are not using JS.