No, that would not be safe. An attacker can replace the HTTP content on the page you're visiting in order to steal your credentials, take over your account, acquire sensitive data about you, or attempt to install malware on your computer.

Please, take a look at this article : https://support.mozilla.org/en-US/kb/mixed-content-blocking-firefox

You can try whether Tracking Protection can block this unsafe content. Tracking Protection is enabled by default in a Private Browsing mode window.

Is this error present on the pages where you login and where you enter your credit card number?

If that is the case then you should contact the website and ask them to look into this matter.

• https://developer.mozilla.org/en-US/docs/Web/Security/Mixed_content
• https://developer.mozilla.org/en-US/docs/Web/Security/Mixed_content/How_to_fix_website_with_mixed_content

Thanks cor-el, I took a look at the page you suggested, https://developer.mozilla.org/en-US/docs/Web/Security/Mixed_content and studied this interesting topic a bit deeper. I displayed the Firefox console on the page that seemed suspicious, and there were lots of warnings about passive mixed content, like this:

Loading mixed (insecure) display content "https://www.xxxx.com/images/xxx/yyyy/zzz.png" on a secure page

I wonder why it warns about https content on an https page - but anyway, all warnings were about images, no active mixed content. What do you think, is it still a good idea not to send sensitive information over this page?