Now, Firesheep was an important, and arguably justified, experiment in that it brought the perils of not using SSL over public wifi (a problem that's been around for years) to public attention by making it quick and simple to demonstrate. However, I fail to see a real justification for this. Of course, people have the right to make and distribute this software, but it feels a bit irresponsible; the source link seems to only superficially warn against illegal use, and the capability to hijack any cookie (instead of just the well-known services) doesn't seem to serve a useful non-illegal purpose.
Why does this app want permissions to modify Gmail messages?
As you need root access to be able to put network interfaces in promiscuous mode and capture raw packets, I think it gets all permissions that are possible. I'm not sure how Android shows this on install, though, whether it still shows all the specific privileges... if so, it would explain this weird privilege.
It's been a few hours since I checked, but I recall it wants only 2-3 permissions, not all permissions. One I remember is "full internet access" and the other is "modify Gmail".