Odd Mozilla Spellcheck

We maintain a wiki page with which DBA will do which maintenance. By convention, the months are the first three characters of the month’s name. In doing updates, I noticed only month marked as misspelled was Sep for September. So Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Oct, Nov, Dec were all okay spellings but Sep was not? Changing it to Sept made it okay.

Weird.