How are you trying to access starbucks, with a bookmark or from a search result in google? It works for me. see screenshot Try downloading Firefox from Mozilla, run it from the folder and see if you have the same issue. https://www.mozilla.org/en-US/firefox/all/#product-desktop-release

I'm not trying to access starbucks.com; I'm AT a Starbucks! I'm assuming the same behavior at any captive portal launch on Earth. Linux Mint-C (current; all updates applied always). I launch FF, it opens with the option to log-in to the WiFi; I click on it, and nothing happens. I disable "HTTPS-only in all windows" and restart FF, and it allows the WiFi log-in.

Sound like you resolved the issue. Their page doesn't like https... How about trying with Firefox from Mozilla to see if you get the same behavior. https://www.mozilla.org/en-US/firefox/all/#product-desktop-release

Also try other browsers to confirm.