I already use the following extensions:
avast! Online Security
WOT (Web of Trust)
No Script
Disconnect
HTTPS Everywhere
Bluhell Firewall (it's not an actual firewall)
Ghostery
I want extensions that will help security and privacy, but not slow down my system too much.
Thank you in advance!
Extensions will not slowdown your system, but in order it will slow your Application(Browser) itself. Over my personal experience, I am currently the Firefox user and I have Issues that I am not bothered. But still I can provide some real tips for optimizing your browser as well as its security.
All those addons are unnecessary. Choose only what is necessary.
Yes you need an AdBlocker firstly and make your selection wisely by choosing a lightweight and application friendly. If I was you I would choose a real time application like AdGuard which would protect all browsers . If you dont wish to have a real time app then you may opt to choose uBlock Origin or AdGuard browser extension which would give you some good responsiveness of you browser and its speed.
Secondly you may keep HTTPS Everywhere which allows you to encrypt the web sites which you visit from your browser.
No requirement of Ghostery because Firefox's Privacy customization would help you and has its own similar kind of feature
And WOT is a good choice too, I guess you how that works.
And My Final List Is:
AdBlocker (uBlock Origin / AdGuard)
HTTPS Everywhere
A Good Password Manager (Lastpass)
A Good Real TIme AV Protection.
With these in the list your browser must work smoothly and efficiently.
If you still wish to customize you browser here is a great tutorial on how to setup it:
A Few Tips to Make Your Firefox Fast