Add SeaMonkey compatibility flag

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Try to install plugin in SeaMonkey


What is the expected output? What do you see instead?
Plugin isnt compatible with SeaMonkey

What version of the product are you using? On what operating system?
HackBar 1.6.2 with SeaMonkey 2.15

Please provide any additional information below.
As SeaMonkey is based on Firefox HackBar should also be compatible with 
SeaMonkey but the install process is hindered on the missing SeaMonkey flag.

Original issue reported on code.google.com by [email protected] on 11 Jan 2013 at 12:47