Replaced taskbar icon?

Hi there,

i have a problem with the Windows 7 taskbar and Mozilla Firefox Portable. After i've tried the prism addon my FirefoxPortable got a new ugly icon. It's really annoying to have new cool themes in Firefox 3.6 but an ugly icon. Is it possible to replace?

What i've tried so far:
- Put an icon set to App\Firefox\chrome\icons\default folder
- Put an icon set toData\profile\chrome\icons\default folder
- Reinstalling firefox
- Clear icon cache (%userprofile%\AppData\Local\IconCache.db)

Do you have any ideas how I can fix the icon?