Please explain the problem in detail. What happens? What is/are the exact error message(s) ?

when I try opening some websites they don't open, and the feedback is that the website does not support encryption for the page I wanna open

Can you open these pages via the http:// protocol ?

Do you possibly have an extension that forces a secure https:// connection or have you forced Firefox to only use HTTPS (dom.security.https_only_mode = true) ?

If you have set the latter then you need to reset this pref on the about:config page.