When you install a new plug-in in Firefox you get a countdown before the actual installation starts. While the countdown is short, I find it unnecessary. If you want to get rid of the countdown and go straight to the installation you can customize Firefox to do this.

In the address bar of Firefox enter about:config

When you get the list of preference, find the one called security.dialog_enable_delay. The value shown is the countdown delay in milliseconds.

Double click on the entry and you'll be able to change the delay. Enter a value of 0 and there will be no delay when installing new plug-ins.