Either add an SSL certificate for static.mangahelpers.com or transfer those files to HTTPS enabled domains
Well, mangahelpers seems to have purchased two SSL certificates some time ago but the implementation of HTTPS has never been completed.
Right now there are certificates for mangahelpers.com and www.mangahelpers.com but the static(.mangahelpers.com ) subdomain is missing one which makes the usage the other two fairly pointless. You should either provide a fully encrypted channel over HTTPS or let it be.
If you can't purchase another certificate than I suggest to stop using static.mangahelpers.com and any other subdomains without a certificate and transfer all filles to a folder on (www.)mangahelpers.com.
Best would be to implement HSTS as well but the above would already be a good start.
P.S.: You may want to start thinking about implementing IPv6, too.
The https was only meant to be used for admin pages. I'll add to rewrite to redirect out of https page.
Could you elaborate what you meant with that sentence? Are you going to remove the public HTTPS implementation? If so, it's a pity since HTTPS has many advantages. If otherwise, a rule can be added to the HTTPS everywhere addon for Firefox.