Whenever you install an extension, Firefox will show the above message box. Initially the Install button will be grayed out for 3 seconds, during which you have to sit there, watching the timer ticking down. Even though there are very good reasons why such a delay is put in force, still some of us have felt the need to remove that.
If you are a Firefox addon-surfer (hey, we coined that term :D) who installs (and obviously, uninstalls) a couple of extensions every day, the 3 seconds delay can easily become a time sink.

Related; Install Addons The Easy Way In Firefox

You can easily disable the installation wait by a simple about:config hack.

  1. Type about:config into the Firefox address bar and hit enter.
  2. Click on "I'll be careful, I promise!" button.
  3. Enter security.dialog_enable_delay into the Filter text box. Double click on that preference value and change the value to 0
Try to install some addons now and you will notice that there is no wait period in the dialog box that pops up.