Information about autoupdate in general is here: https://support.mozilla.org/en-US/kb/update-firefox-latest-version?esab=a&s=auto+update&r=5&as=s

The main pros is your browser will always be as secure as we can make it, you'll get new user features and also help move the web forward by ensuring that web developers can start taking advantage of new open, standard web features.

There are few cons, especially with silent/background updates in place. The main one would be if you require a very old plugin that is not being updated, however, realize you are putting your computer at risk if you choose to do this.

Pros: Getting security patches as soon as the come out helps protect your web browser from new exploits.