1.THis extension help you to finding hidden api keys in websites.
https://addons.mozilla.org/en-US/firefox/addon/trufflehog/
2.THis extension is for using burpsuite or any other proxy capture (mitm) application.
https://addons.mozilla.org/en-US/firefox/addon/foxyproxy-standard/
3.THis extension will help you to find which technology running on website.
https://addons.mozilla.org/en-US/firefox/addon/wappalyzer/
4.THis extension used for tempmail service in fast and easy way.
https://addons.mozilla.org/en-US/firefox/addon/temp-mail/
5.THis extension used for finding all email from the website best use for submitting report on public programs..
https://addons.mozilla.org/en-US/firefox/addon/hunterio/
6.THis extension give you all useful paylaod for testing site in easy way.
https://addons.mozilla.org/en-US/firefox/addon/hacktools/
7.THis is best extension for cookie editor also this will tell you site http only or secure flag flag set or not.
https://addons.mozilla.org/en-US/firefox/addon/edit-cookie/
another alternative:
https://cookie-editor.com/
8.THis extension is best use for protecting your vpn ip from webrtc exposer.
https://addons.mozilla.org/en-US/firefox/addon/happy-bonobo-disable-webrtc/
9.THis extension used for fetching all domains and links from websites and google results.
https://addons.mozilla.org/en-US/firefox/addon/link-gopher/
10.THis extension used for finding hidden possible parameter or secret keys.
https://addons.mozilla.org/en-US/firefox/addon/findsomething/
11.THis extension will help you to find .git files in website easy p1 Information disclosure.
https://addons.mozilla.org/en-US/firefox/addon/dotgit/
12.THis extension used for opening multiple sites in one time.
https://addons.mozilla.org/en-US/firefox/addon/open-multiple-urls/
13.THis extensioin is best use for blocking irritating ads or trackers on sites.
https://addons.mozilla.org/en-US/firefox/addon/ublock-origin/
14.THis extension will protect your eyes better use for night time while hunting.
https://addons.mozilla.org/en-US/firefox/addon/darkreader/
15.THis extension used for switching user-agent better for testing site with many user-agents.
https://addons.mozilla.org/en-US/firefox/addon/uaswitcher/
16.THis extension tell you all vulnerable javascript library.
https://addons.mozilla.org/en-US/firefox/addon/retire-js/
17.THis extension is used to translates all languages in websites.
https://addons.mozilla.org/en-US/firefox/addon/traduzir-paginas-web/
18.THis extension fetch all urls from waybackmachines same like waybackurls
https://addons.mozilla.org/en-US/firefox/addon/waybackurl/
19.SponsorBlock lets you skip over sponsors, intros, outros, subscription reminders, and other annoying parts of YouTube videos.
https://addons.mozilla.org/en-US/firefox/addon/sponsorblock/
20.The Shodan plugin tells you where the website is hosted (country, city), who owns the IP and what other services/ ports are open.
https://addons.mozilla.org/es/firefox/addon/shodan-addon/