The Web Developer extension adds various web developer tools to the browser.

https://addons.mozilla.org/en-US/firefox/addon/web-developer/

Ads just annoy me and I've never bought anything from an ad.

On YouTube, you have to listen then, no way, I will not waste minutes of my life waiting for ads.

AdBlocker for YouTube™ removes all annoying Ads from YouTube.
Important features:
1. Allows you to remove annoying contents from your YouTube videos.
2. You can also remove annotations from videos and musics (see options page).
3. Loads videos and YouTube website faster.
4. Supports both Firefox Desktop and Mobile (Android).

https://addons.mozilla.org/en-US/firefox/addon/adblock-for-youtube/