On the machine that I'm currently using, the browser is Firefox 56.0 (32-bit), which is running on a Linux OS.
Note that Safari on an iPad mini does not produce this warning.
The Firefox browser does have various plugins, but none inject mark-up on any pages and it is fine on other secure sites.
The pages I did my testing on are the forum index page (index.php) and the FAQ page (app.php/help/faq).
On the index page, the only non https:// links that I can see are:
http://spectrumcomputing.co.uk/ (which is what you get if you click on the logo)
As this forum does not allow attachments, I can't directly attach images showing the problem.
If you want to see the images, let me know and later I will post them up elsewhere and link to them (I wonder if that will also add to the problem ).
Broken link? Feature request? Anything related to the Spectrum Computing website here.