What are your favourite add-ons/extensions on Firefox?

I'm sorry. I should have looked in this community before posting. I forgot.

I found 2 similar threads. I'll leave the links here:

1. Best Firefox Extensions? by MokMokFIsh
2. What are your favourite add-omd for Firefox? by t0fr

Edit: Your link was not working for me. I don't know why. It's same as the 2nd link that I posted.