In bug 1268340 we changed app.update.promptWaitTime preference to 8 days with the following rationale: "The current value is 2 days. This bug will make it so the update dialog asking the user to restart to apply an update is displayed after 8 days. 8 days was chosen since some users keep their browser open all week and restart over the weekend and this will make it so those users don't see the dialog." On Nightly, the current value is 12 hours which is IMO way to short and is also the feedback that nightly user tell me at events around Nightly There is probably a middle ground between 12h and 8 days, I think that having relatively up to date nightly users (a build of the week) is of great value and if we want to ship Nightly more often (bug 1349227) yet be less intrusive with our updates, that value should be more than a day. I propose that we change that value from 12h to 3 days. 8 days wouldn't help having a large pool of up to date users and 2 days would be too short to avoid getting the Upgrade dialog on the Monday morning after the week end.

The value for Nightly was intentional since on Nightly there is a desire to get clients to update sooner so that we can get crash reports, etc. from clients. The big update dialog will be going away in the near future on Nightly anyway in bug 893505.