Depending on how you have your Firefox setup to handle updates (Firefox Button > Options > Advanced > Update), you will be prompted when there is an update for Firefox. You have the option of doing the update then or to have Firefox remind you later. The problem with later is it is 24-hours later which still may not be a good time to stop what you are doing to shut down Firefox (albeit the update only takes a couple minutes to install).  You can change the frequency of the reminder by changing the value for ‘later’ via an about:config tweak. Keep in mind the value is in seconds with default being 86400 seconds or 24-hours.

• In a new tab type about:config and press enter
• Filter for app.update.interval
• Double click and enter a new value in seconds (6 hours would be 21600; 12 hours would be 43200; 36 hours would be 129600)
• Click OK then close the tab

Now, I am not sure what happens if you make this change after Firefox has already alerted you to the update and you asked to be reminded later. My thoughts are the new interval won’t go into effect until next time you are prompted for an update and select to be reminded later. There are two other possibilities, the timer resets as soon as the change is made or the current timer is shortened (or extended) based on the new value. The next update is scheduled for this Tuesday, August 28st, I’ll see if I can mess with this some on one of my sandbox profiles.

One final tip/reminder, you don’t have to wait until you are ‘reminded’ to do the update. You can at anytime go to Help > About Firefox and opt to install the latest update.