From MozillaZine Knowledge Base
This preference determines if the application will show up a dialog box at restart to inform the user an upgrade was applied successfully.
Possible values and their effects
The dialog box will be shown. It's the default for all applications but Firefox.
The dialog box will be not shown. It's the default in Firefox, because it shows a start page to inform the user an update was applied.
First checked in
Has an effect in