How we can enable squid to cache web content (let says from firefox) for SSL connection, i mean for https URLs?
Actually SQUID can be used to access HTTPS traffic - it is in essence a man-in-the-middle attack - and there are caveats:
See: http://wiki.squid-cache.org/Features/SslBump
I have not tried cacheing this data yet, so can't say that it will work with absolute certainty. If/when I do, I'll update this post.