I found this issue while using FF 40.0.3 on Ubuntu 14.04, but I replicated it on a VM of 15.04 with Firefox 37.0 (default setup for Ubuntu's liveCD). As well as FF 39.0 on Windows 7
- Logged in as a regular user, go to https://secure.phabricator.com/maniphest/task/create/
- enter random title and random description (this is irrelevant as we are going to fail to post anyway)
- as 'Visible To' select 'Space S1: Core' and 'Administrators'
- as 'Editable By' set 'Administrators'
- create task (you will be taken to a page with "unhandled exception")
- press back button in the webbrowser or on your mouse (just go back)
- done (all the text you entered is most likely gone)
This does not happen in Chromium on Ubuntu, neither does it happen to Chrome on Windows. In those cases your task stays prefilled with data you were trying to submit.