If you click on the Security tab, you can view, from the checkboxes, any of three instances from which Firefox may warn you.
If you click Settings in the "Warning Messages" block, you can determine which messages you do or do not wish to see.
Your WiFi may be warning you about a site with unsigned credentials, or insecure or undocumented encryption.
As far as our experts here can tell, mangafox does not appear to be malicious itself, but I cannot validate all the sites represented by the hundreds of links on mangafox.
