We found that a Google Form that was restricted to “Middlebury users only” was having issues displaying in Safari and Firefox. It returned a 403 error. It worked fine in Chrome. When it was open to the world, it worked in all browsers.
The issue was the way in which the browsers treated cookies.
In Firefox, if you go to Preferences>Privacy & Security and then create a custom setting with the Cookies box unchecked, the form will then embed correctly even while restricted to Middlebury community members.
For Safari, you’d have to go to Preferences>Privacy and uncheck the box that says “Prevent cross-site tracking.”
I think the issue is that the cookie saying you’re logged in and a member of Middlebury is from Google and the Google form looks for the login cookie but since it is on the instructure.com site it sees that cookie as cross-site and blocks it for privacy reasons. This is a way to make it work, but probably not worth the hassle. We ended up just putting it as an external link in the sidebar.