It is possible to open Web Clips in Google Chrome, Puffin and Firefox:

Google Chrome:

You can add a Web Clip that starts with "googlechrome://" followed by the web address. This will open Safari and afterward redirect it to Google Chrome.

Note: If you want to force a website in HTTPS you can use "googlechromes://" followed by the web address.

Firefox:

You can add a Web Clip that starts with "firefox://" followed by the web address. This will open Safari and afterward redirect it to Firefox.

Puffin:

You can add a Web Clip that starts with "puffin://" followed by the web address. This will open Safari and afterward redirect it to Firefox.

Note: If you want to force a website in HTTPS you can use "puffins://" followed by the web address.