(see http://blog.mozilla.com/addons/2010/12/02/toolbar-buttons-in-firefox-4/)

NOTE: This text ONLY applies to the icon displayed to the side of the addon name in the Add-ons Manager, whether in list view or detail view.  You do NOT provide a larger 'preview' image bundled with your addon (be it an extension or a theme); the big image you may see when you go into an addon's detail view seems to be magically downloaded from addons.mozilla.org - Firefox probably just grabs the first 'preview' image for the addon.
... EXCEPT when it comes to themes.  :-)  They're a special case, and Firefox will look for the file 'preview.png' in the same directory as it's looking for
the icon.png and icon64.png files (ie. the same root dir as install.rdf), and display that as the theme's large preview image if found.  See source:
http://mxr.mozilla.org/mozilla-central/source/toolkit/mozapps/extensions/XPIProvider.jsm#6622
^ source was checking for 'preview.png' file at time of writing, 2011-03-06.

The Add-ons Manager Icon
========================
On Firefox 3.6 and earlier versions, the Add-ons Manager displays a 32 x 32 px icon for every installed add-on. Traditionally, this icon was defined using the iconURL tag in the install.rdf manifest. This is a chrome URL, so it has the disadvantage that it doesn’t work when the add-on is disabled, or when the install dialog for the add-on is displayed. The solution for this was to create a default location for the icon, so that it could be extracted without having to install the add-on and resolve the chrome URL. This was introduced in Firefox 3.6, so instead of the iconURL tag, you could just place a file named icon.png in the root of your package and it would work.

Now, on Firefox 4, the Add-ons Manager has an entirely new face. Since the UI resides in a tab instead of a small window, some elements like the add-on icon can now be larger.

The new manager interface has 2 icon sizes: 48 x 48 px for the list view and 64 x 64 px for the detail view. The detail view is what you see if you double-click any add-on on the list (at the moment the detail view scales the icon down to 48px, bug 615980). So, now we also have an icon64URL tag in install.rdf and an icon64.png file we can use for the 64px version of the icon. The default version should be 48px.

There is no perfect way to support both Firefox 4 and earlier versions. I recommend that you switch to the 48px and 64px icons, which means that on earlier versions of Firefox the 48px icon will be scaled down to 32px. Not ideal, but also not terrible. Another possibility is to use both the default icon files and the chrome URLs. If both are present, the chrome URLs take precedence (when possible), and they are flexible enough to point to different files on different versions of Firefox.
