From the website, you will want to click the link "View PetCam":

After clicking the link you will be redirected to the Amcrest login screen:

Click where on the link "Please Click here to download and install the plug-in"

Firefox will automatically install the browser plug-in, asking you to restart the browser.

Restart Firefox, then log in as you had done previously.  Firefox will ask if you want to allow the app, click "Yes".

Once you have done that you will see the webcam feed: