Is this an embedded rectangle in the first site, or does it open in its own tab? Sometimes an embedded page will show an error message about how the other site doesn't allow its content to be embedded (framed). To work around that, try this: right-click the error, then expand "This Frame" and click "Open Frame in New Tab." Hopefully that will work. If the page is opening in its own tab already, what error message are you getting there? If it's a secure connection problem, could you copy/paste the error code into a reply (here at https://support.mozilla.org/questions/1400169 -- not by email). It usually is in all caps and has underscores, like SEC_ERROR_SOMETHING_WEIRD. Sometimes you need to click an "Advanced..." button to get the code. Spoiler alert: we'll look it up in this article: [[How to troubleshoot security error codes on secure websites]].