I regularly like to "Pin as App Tab" SquirrelMail in my browser for easy access. When this is done, the only identifying marker is the site icon. SquirrelMail doesn't currently have one.
Adding this to SquirrelMail turns out to be easy:
Right after the line containing <head> add the line:
<link rel="SHORTCUT ICON" href="../SquirrelMail.ico">
Then copy the attached SquirrelMail.ico file to the root of the SquirrelMail installation.
Refresh the SquirrelMail page and an easily identifiable icon will appear. Now right click the tab and select "Pin as App Tab" (what Firefox says). The tab scoots over to the far left of your tab list, and now the familiar Squirrel is showing. My users like this feature... and so far it works in Firefox, Chrome, and MS Internet Explorer. I haven't tried any other browsers.