British-English Dictionary for UK Spell checking

https://addons.mozilla.org/en-GB/firefox/addon/british-english-dictionary-2/

I Don't care about cookies - Hide cookie confirmations on websites

https://addons.mozilla.org/en-GB/firefox/addon/i-dont-care-about-cookies/

uBlock Origin - Popup and Ad blocker

https://addons.mozilla.org/en-GB/firefox/addon/ublock-origin/

Measure IT - Ruler addon to measure width/height of areas on websites

https://addons.mozilla.org/en-GB/firefox/addon/measure-it/

Web Developer - adds various web developer tools to the browser.

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

ColorZilla - Advanced Eyedropper, Color Picker, Gradient Generator…

https://addons.mozilla.org/en-GB/firefox/addon/colorzilla/

Fireshot Lite - Full webpage screenshots or saving as PDF

https://addons.mozilla.org/en-GB/firefox/addon/fireshot/

HTML Validator - Validates HTML of websites

https://addons.mozilla.org/en-GB/firefox/addon/html-validator/