Is there any way to change Firefox system icon (the one on the left top of the window)?
Precision : I want to change the icon of a bundled version of Firefox with apache/php and my application. So manual operation on each computer is not a solution. I try Resource Hacker and it's the good solution. The add ons one is good too.
I think you mean the system icon, not the site icon as someone else thought. On a Mac, you can hold-Click -> Get Info on Firefox.app, then drag or paste an image on top of the icon.
I'm not sure about Windows, but I think you may need to compile from source to change it.
If you're talking about the application icon (which under Windows is typically located in the top-left corner of the application's window), then... no... and yes.
Like most windows apps, the icon you see there is probably a resource compiled into the application itself, so you can't change it.
There may be add-ins to Firefox that let you do this, but I doubt it - that icon is trademarked and "identifies" the Firefox "brand" (if you will). So it's unlikely that you could change it at run-time.
Firefox is open-source; you could always just download & compile your own version, replacing the icon resource with your own. A bit dramatic, but possible.
Resource hacker does the job of swapping application icons in Windows (up to XP, not tested with Vista yet).
Available at: http://www.angusj.com/resourcehacker/
There are icon packs available at http://iconpacks.mozdev.org/ that work by installing an extension. If you want to use your own icon, extensions are just zipped files so change the extension from xpi to zip and examine the source code and images it contains to customize it. If you do customize it, I suggest changing the GUID that so it doesn't auto-update and overwrite your customizations.