After searching for hours, and trying everything, I figured out how to get two profiles running off different icons in the taskbar (ie. different icons and so that they don't combine).
This is all in Win7, and make sure you are careful to use the quote marks I've used in the 'type this' lines. They make a difference. Type everything in bold exactly as is.
Firstly, create a new firefox profile.
Go to Start Menu Run and type: Firefox.exe -profilemanager
Create new profile called Mail for use with mail clients etc (or whatever you want to use it for).
Open cmd prompt as Administrator (In Run type cmd then right click CMD and open as admin)
Type this then hit enter:
mklink /H "C:\Program Files (x86)\Mozilla Firefox\fmail.exe" "C:\Program Files (x86)\Mozilla Firefox\Firefox.exe"
Type this then hit enter:
mklink /D "C:\ProgramData\Mozilla Firefox" "C:\Program Files (x86)\Mozilla Firefox"
Create a new link by right clicking anywhere on desktop and choose New->Shortcut
Type in Target: "C:\ProgramData\Mozilla Firefox\fmail.exe" -p Mail -no-remote
Type in Start In: "C:\ProgramData\Mozilla Firefox"
Now click Apply then Ok.
Right click it, then Properties.
Change Icon to Standard Windows Mail icon (just select any random file, and if it doesnt find one, the default ones will come up. The Mail icon is on the top about halfway along)
Click Ok - Apply - Ok
Move this shortcut to the taskbar by dragging it onto it.
You should now have a mail icon in the taskbar.
Right click it, then right click fmail.exe then Properties
Change icon to the same Mail icon you just used previously.
Click ok - apply - ok.
Go to Start Menu Run and type: %localappdata%
Delete IconCache.db in the folder that pops up
Go back to black and white cmd window
Type & hit enter: taskkill /f /im explorer.exe
Type & hit enter: explorer.exe
Now right click the firefox icon in the taskbar and right click firefox.exe then properties
Type in Target: "C:\Program Files (x86)\Mozilla Firefox\firefox.exe" -P default -no-remote
Now you can open two profiles simultaneously, with different icons on the taskbar!!
Open the Mail icon, then CHANGE THE APPEARANCE/THEME under add-ons so you instantly recognise which one you're in by choosing a very different theme to your main browsing profile.
Don't forget to add the usual minimum add-ons (BetterPrivacy, Redirect Remover, Adblock Edge, Self-Destructing Cookies) to the new profile as it is a completely blank profile.
PS> If you are still having issues with icons combining on the taskbar, Run -> regedit, and find "HKEY_CURRENT_USER\Software\Mozilla\Firefox\TaskBarIDs"
Then create a new string key called "C:\ProgramData\Mozilla Firefox" and use the same data variable as in the standard one that's there, but
change the very last character to anything else. This will stop the icons on the taskbar combining.
PPS> If you're still having issues, make sure you type in EXACTLY what I did above in bold INCLUDING THE QUOTE MARKS!!!!