Apple stopped releasing Safari updates for Yosemite years ago. Many modern websites (YouTube, banking portals, Google Docs) will either refuse to load or will display broken layouts on the legacy Safari version. Hence, Firefox for Mac 10.10.5 remains the best bridge to the modern web.
While Apple has long since moved on from Yosemite, your hardware might not have. Many trusted Macs from 2007 through mid-2014 are stuck at this operating system ceiling. Unfortunately, Google Chrome and Safari have dropped support for this OS, leaving users vulnerable. firefox download for mac 10.10.5
You need the Extended Support Release (ESR) channel. This is the final version of Firefox built for older Intel Macs running Yosemite. Apple stopped releasing Safari updates for Yosemite years
This is the official, safe repository of every Firefox build since 2004. While Apple has long since moved on from