What is your favorite browser extension?

My favorite browser extension has to be Web Boost. It actually increased the speed of web page loading by double. I used to wait quite long for a certain web page to load but after adding this extension to my browser, it immediately loads up with all content on it. 

Yahoo! e-mail used to take a full minute to load and now, it loads in a couple of seconds. It's a real time saver.

Mostly I don't use any browser extension. But in firefox I am having one extension for downloading flash and online videos. It is quite useful to download flash file and videos. There are other ways to do the same but this one is quite faster and useful. So I am using this only extension.

I have 3 essential extensions, Adblock Plus (Chrome), ublock Origin (Firefox) and VideoDownloadHelper. To me all this three extensions are essential to anyone that wants to avoid unwanted ads and if you need to download media content from places like Youtube or Vimeo / Dailymotion

Alexa Toolbar as I do research into SEO and the toolbar helps me check global rank, country rank etc

Adblock plus is my must have

tampermonkey also is one of my must haves

Start.me is a great browser extension.
I also run uBlock and AdBlock Plus