Gmail auto-saves your content, JIRA should do the same (or have an option to). The Safari back "feature" is not much worse than closing a window, browser crashes or OS crashes - stuff happens, a simple modal to prevent loss isn't adequate.

I'm not trying to diminish the complaint of the posting (which is valid) but web-apps should really capture "intents" as new stored data. If I intend open a new ticket, a new record should be automatically be created regardless, and then marked "deleted" if I abandon it (which could be reversed later).