My default browser app is set to Firefox already and I have disable the permission that allows mixplorer to open links
The only solution I found is to delete mixplorer but I can't do that everytime I need to open a link on the internet
Long pressing on an .html file then opening menu should show the "open with" option. If you are not seeing that then you might have another more fundamental problem. Note: those menu options can be arranged and hidden in the Buttons settings. Make sure they are visible (enabled).
This might (or might not) reset the association without having to reset much else.
Menu > More settings > Reset > Remember.