FoxyProxy
Wappalyzer
Whatruns
Owasp Pentester kit
LinkGopher
Cookie Quick Manager
Firefox Relay
Open Multiple URLs
pwnfox [Best for IDOR]
Multi Account Container [Firefox]
google translate
Hackbar
Max HacKBar
DOM Invader [Burp Browser Extension]
Warning
Some of these extensions are currently only compatible with Mozilla Firefox; however, there's a possibility that they could become available for other browsers in the future.