I use firefox 16 (previous version) this way:

cd ~/OldFirefox/firefox16
./firefox -no-remote -P firefox16

using a separate profile, (because version 17 broke an important add-on I need called Wikilin), but I want to do all my "normal" browsing using the up to date version.

The only problem is that when both are up and running, the icons in the panel / taskbar are the same, and I don't want to (or have my family members) mistakenly use the unsecured one for browsing.

I tried this:
./firefox -no-remote -P firefox16 --icon /home/david/Dropbox/Photos/Notes.png

and I tried changing all the applicable icons I could find in the ~/OldFirefox/firefox16 folder

Is there any way to force an app to use a specific icon? Any other ideas?