I have a few that I find essential:
- AdGuard: Free ad-blocker
- Customize Search Engine: Does what it says on the tin
- Hush: Removes pesky cookie banners and suchlike
- JavaSnipt: JavaScript blocker
- AMP Shockwave: Redirects AMP pages to their original versions
- Sideways: Lets you rotate webpages regardless of screen orientation
- Userscripts: Adds support for user.js scripts, such as Privacy Frontend Redirect and Return YouTube Dislike
- Vinegar: Replaces custom video players (such as those used by YouTube) with the default HTML5 video player, which also enables background play and removes YouTube ads
- Web Inspector: Also does what it says on the tin