We are describing here a step by step process to install MetaMask
Open the browser of your choice (we'll be using Firefox in this example) and go to https://metamask.io/
Please click on the Download button
Click on "Install MetaMas for your browser"
After clicking on "Install MetaMask for Firefox" you'll be redirected to the Addons page for Firefox where you'll have to click on "+ Add to Firefox" button
Click on "+ Add to Firefox"
Please click on Add as shown below
Click on "Add"
Click on "Get Started" to start using MetaMask