Apparently the wireless network here is causing the issue. When you connect to the wireless network, you first have to go to an unencrypted website (such as http://example.com), after which you're redirected to a login page (captive portal). However before you have entered your credentials, any connections to https websites (or any port other than 80) will fail with a timeout. Linux does 5/6 attempts at establishing a connection (TCP SYN) it seems, and all of these have to timeout before Firefox understands it cannot connect to whatever server it's trying to securely connect to. This is what's taking so long.

Solution: killall firefox; sudo ifdown wlan0; firefox&; sudo ifup wlan0

Apparently the wireless network here is causing the issue. When you connect to the wireless network, you first have to go to an unencrypted website (such as http://example.com), after which you're redirected to a login page (captive portal). However before you have entered your credentials, any connections to https websites (or any port other than 80) will fail with a timeout. Linux does 5 or 6 attempts at establishing a connection (TCP SYN) it seems, and all of these have to timeout before Firefox understands it cannot connect to whatever server it's trying to securely connect to. This is what's taking so long.

Solution: killall firefox; sudo ifdown wlan0; firefox & sudo ifup wlan0