Why is the connection insecure displayed differently in the browser on different sites?

Question

Joomla

I go with Firefox to different http-sites. For some reason, on some sites, the browser at the beginning of the address bar simply shows i in a circle and if you click, it says "Insecure connection". And on some sites, it also shows a crossed-out padlock next to it, and when opened, under the inscription "Insecure connection" it is also written that logins can be compromised. How does the browser determine this and how to deal with it if the site is mine? Just installing an SSL certificate? It would suit me for now to stop showing the lock, that is, to protect logins. Site on Joomla, I don't know if it matters.

Answer 1

Show example sites.