" Change the Firefox3 ActivityIndicator (Throbber) "

at the moment just find the <chrome folder> 
+ also open the folder with the chosen animation (Rotor or Pulse) then follow the READ ME instructions.

BTW you can also change the throbber position:  open the "customize toolbar" dialog and drag it on the toolbar that you prefer (for the center position look at the image enclosed)

Enjoy !!

frankell
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" Finding the chrome folder " 

It's very simple for the portable version: FirefoxPortableTest\Data\profile\<chrome>
  
Otherwise (source: http://kb.mozillazine.org/Profile_folder_-_Firefox ):
the installation directory includes a folder named "profile" (for example, C:\Program Files\Mozilla Firefox\defaults\profile on Windows), but this folder contains program defaults, not your user profile data. On Windows 2000/XP/Vista and on Linux, the folder containing your user profile data is hidden by default and you will need to show hidden files and folders to navigate to the profile folder. 

Windows 95, 98, and ME
Profile folders are in one of these locations: 

C:\Windows\Application Data\Mozilla\Firefox\Profiles\profile folder\<chrome>
C:\Windows\Profiles\<Windows login/user name>\Application Data\Mozilla\Firefox\Profiles\profile folder\<chrome>

Windows 2000 and XP

You can also navigate directly to your folder at the following path: 
C:\Documents and Settings\<Windows login/user name>\Application Data\Mozilla\Firefox\Profiles\profile folder\<chrome>
The Application Data folder is a hidden folder; to show hidden folders, open Windows Explorer and choose "Tools ? Folder Options ? View (tab) ? Show hidden files and folders". 

Windows Vista
 
You can also navigate directly to your profile folder at the following path: 

C:\Users\<Windows login/user name>\AppData\Roaming\Mozilla\Firefox\Profiles\profile folder\<chrome>. 
The AppData folder is a hidden folder; to show hidden folders, open a Windows Explorer window and choose "Organize ? Folder and Search Options ? Folder Options ? View (tab) ? Show hidden files and folders". 

Linux and Unix
Profile folders are located here: 

~/.mozilla/firefox/<profile folder> 
The ".mozilla" folder is a hidden folder. See this article for more information. To show hidden files in Nautilus (Gnome desktop's default file browser), choose "View -> Show Hidden Files". 
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