Sadly this is not testable in the beta version. But it is in the Alpha version 3.6, below explains you how to do it. Be aware that this fix is completely at your own risk, as 3.6 of it is not stable yet.

Step 1
http://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/latest-trunk/ Download Minefield. This is the project name for alpha releases of Firefox. On that page you can download it, for windows that would be firefox-3.6a1pre.en-US.win32.installer.exe

Step 2
Install minefield. It will not overwrite your existing version of firefox, but wil copy your bookmarks , add-ons, passwords etc.

If you go to pixeljoint.com right now in your new Minefield browser you can already see the images crispy when you zoom in. This because it seems PJ already added the right tags to its css. Thanks!

Step 3
To get the pixel zooming on every website you visit (such as wayofthepixel.net) we need to take another step.

We must create a file named userContent.css
This CSS file will apply the code in it to every page you visit.

In windows, this page must be created in %appdata%\Mozilla\Firefox\Profiles\profilename\chrome

You can find it by browsing to C: > Documents and settings > ‘Username‘ (mine is Daniel) > Application Data.
Application Data is a hidden folder, you can unhide it by going to (in your explorer) Tools > Folder Options > View > Show hidden files and folders
http://www.bleepingcomputer.com/tutorials/tutorial62.html

Non windows users must figure it out themselves, but the file will always be located in Profilename\Chrome

Now open notepad or a similar text editor, and paste in this code:

img[src$=".gif"] { image-rendering: -moz-crisp-edges; -ms-interpolation-mode: nearest-neighbor; }
img[src$=".png"] { image-rendering: -moz-crisp-edges; -ms-interpolation-mode: nearest-neighbor; }

Save it as userContent.css (and watch out it does not become a .txt but a .css) in the chrome folder

Thats it. You are done! When you click images on any site to zoom in, they will be shown ina ll its pixel glory (after you restart your browser).

If you have any questions, comment!