When you click on a XPI file (install button in Firefox addon page for an extension) to install to install a Firefox extension, you would see a counter counting down from 3 to 0 before letting you to install. This is one of the most annoying settings in Firefox that I have ever come across.
This was delibrately done to avoid known security issues explained in detail here.
1. Open a firefox browser window and type “about:config” in the address bar.
2. Click “I’ll be careful, I promise” button.
3. In the Filter search box, enter “security.dialog_enable_delay“. You can see this set to 2000 milliseconds.