Issues with Featured Content images on high resolution screen (Firefox)

Hello, I just created a new website www.graceplainfield.org. In testing I found an issue with featured images on high resolution screens in Firefox. Some of the images are sizing too small in both the featured content section and at the top of the related page. I only experience the issue in Firefox at high resolution (Microsoft Edge and Chrome look fine). Firefox is also fine on lower image screen. Error happens in these conditions: - Dell XPS 13 with resolution 3200 x 1800 (250% scaling) - Windows 10 - Firefox 44.0.2 No issues on iPhone, iPad, other browsers, etc. Any suggestions? Much appreciated