Does Firefox update despite being set to "never check for updates"? This might be why.
If, like me, you have set Firefox to "never check for updates" for some reason, and yet it does sometimes anyway, this could be your problem: the chrome debugger.
The chrome debugger uses a separate profile, with the preferences copied from your normal profile. But, if your prefs (such as
app.update.enabled) have changed, they remain in the debugger profile as they were when you first opened the debugger.
App update can be started by any profile using the app, so the debugger profile sees the pref as it once was, and goes looking for updates.
Solution? Copy the app update prefs from the main profile to the debugger profile (mine was at
~/.cache/mozilla/firefox/31392shv.default/chrome_debugger_profile), or just destroy the debugger profile and have a new one created next time you use it.
Just thought you might like to know.