If you have given a browser extension permission to do so, it can access all data on all pages you use, including MyEtherWallet. You're placing trust in the browser extension.
You can increase your security by only accessing MyEtherWallet on a browser without extensions, for example Chrome's incognito mode.
The only way to be 100% certain of your security is to generate your private keys with dice rolls (or another physical method that you know to be random). Only enter the private keys on a computer without network connection and without wireless networking capabilities (e.g. WiFi, Bluetooth, NFC, infrared). Also make sure no other people or cameras are watching your screen, keyboard or dice. Also, make sure no microphones are listening to your typing patterns, because on most keyboards every key has a slightly different sound. (you can probably hear it)
If you're not doing all that, you're placing trust in the manufacturer of your hardware, your operating system, your browser, browser extensions, the software you use to generate private keys and other software running on your computer.
It's a matter of compromising between security, trust and convenience.