User Agent: Mozilla/5.0 (X11; Fedora; Linux x86_64; rv:39.0) Gecko/20100101 Firefox/39.0 Build ID: 20150708164734 Steps to reproduce: Trying to disable the "It looks like you haven't started Firefox in a while ..." prompt. Actual results: It, like the cat, keeps coming back. I never want to reset, and have accidentally hit the big "reset" button, causing a loss of configuration/work. So have friends. Expected results: There ought to be a way of disabling this recurrent prompt. A "don't ask again" gui button in the notifier would be great. At least, expose the internal browser.disableResetPrompt boolean in the perference guis, or the about:config list.
I'm not sure a GUI will be added for this, but if you have problems with it coming up more often than it should, perhaps you could check the dependencies for bug 498181 to see if it's a known issue and if not, try figuring out what causes it to come up?
It's not obvious which of the possible causes of that notice are actually triggering in our case. The notice doesn't indicate why it thinks a reset is worth suggesting - that makes it just all the more annoying.
Some of the dependent bugs' comments provide some insight into this (e.g. a failure to update mtime of a certain file in the profile can cause this IIRC). I'm not familiar with this issue myself, so can't be of more help.
It looks as though I'm experiencing the same thing (bug 1192175).