I find it irritating when Firefox checks for browser and addons updates whenever it is started. Actually it does this once a day. While I would like the update check, I would like it to be more spaced out, let us say every 15 days.
To do this, first ensure that udpates are enabled. They can be enabled at Tools → Options → Advanced → Updates.
After this, open up about:config to set the intervals. The interval periods are integer values, set in seconds. So 15 days is 1296000 (seconds).
To set the application (Firefox) update interval, set the value of app.update.interval to 1296000.
To set the addons update interval, set the value of extensions.update.interval to 1296000.