when following an email link. I usually get just any past seen email onscreen for 15sec.

Use this addon to open email links with the email app itself: https://addons.mozilla.org/en-US/android/addon/open-native-app/ Beyond that... you could use Stylish addon to load a cleaner, less cluttered version of the site from Userstyles.org. Maybe don't delete cache and offline data etc in privacy options if you are.