From MozillaZine Knowledge Base
A user can choose to tell the application to remind them later about how they need to restart for a downloaded update to take effect. This preference controls the amount of time before the application tells the user again about restarting.
Possible values and their effects
The number of seconds until the user is reminded again about restarting. (Default: 1800, or 30 minutes; in Firefox 3 and above, 86400, or 1 day)
First checked in
Has an effect in