Chrome has done the same. Likely due to the amount of malicious ads hosted there. Generally you shouldn't use websites that use Adfly as it's nothing more than a service which spreads malware in exchange for ad revenue.
I'd highly recommend moving Optifine's downloads to Curse or Github. I've always used the mirror so that I can avoid Adfly and I can only assume everyone else does as well. Using the Adfly links never actually provides a download of Optifine anyway. It just downloads some shady installer.