Seamonkey 2.5 is my default program, so I use it daily. And, you are correct. I did see 11 backup file in one of my three profiles of Seamonkey, as well as in one of my Firefox profiles. My Firefox profiles are in use, as each is associated to a different version of the program, but I am using only one Seamonkey version, at this time.
The Seamonkey profile in use has consecutive days' backups. The Firefoxes which are opened daily have some older backups, but I am not opening them daily. None of the profiles had more than 11 files.
I deleted many of the backup files, so I cannot make a definite statement that only 11, and not 10 files would be in the backup folder, but, as I said, Seamonkey uses only one profile, and it is default.
UserAgent: Mozilla/5.0 (Windows NT 5.1) AppleWebKit/535.2 (KHTML, like Gecko) Chrome/15.0.874.121 Safari/535.2