By default, when you are installing an extension in Firefox, you have to wait for a few seconds to continue
If you don’t want to wait, simple do as following:
- Type about:config in the address bar
- Search for security.dialog_enable_delay and set it to 0
- Save & restart Firefox
There’s an extension that helps you to do all that stuff, named No XPI Install Delay, you can check it out here: https://addons.mozilla.org/en-US/firefox/addon/no-xpi-delay/

