For instance I tried to update my prime video, and Amazon tells me I have to go to the website to manage my subscriptions. So I try to go to amazon.com and Firefox, and it opens the app instead of the website. So how do I get to the website so I can manage my subscriptions?
Assuming you're using Firefox as a mobile browser. Hit the three little dots on the top right, scroll down and hit settings. Near the bottom under advanced there should be an "Open links in apps" option. Set it to never.
Huh? You're opening your browser, and going to amazon.com and it redirects you to the app?
I'm assuming you're on mobile. Try using your browsers incognito/private browsing feature- you'll probably have to login again but it should prevent app redirects like that.