Can Firefox's "view source" be set to not make a new GET request?

If you install the web developer toolbar extension, there's an option under "View Source" called "View Generated Source" which will show you the current source of the page, including any DOM changes you may have made.

You do a Ctrl+A, right click and "view selection source", that doesn't re-request the page.

Use the FireBug extension. It displays (and allows you to navigate) only the rendered source, so there is no need for another request (and it shows Javascript changes).