Yosemite lacks the modern system-level privacy controls found in Big Sur or Monterey. Firefox compensates for this with built-in . It automatically blocks third-party trackers, cryptominers, and fingerprinters that Safari on 10.10.5 would let through.
If you can access the internet, the URL structure for Mozilla archives usually follows this pattern: https://ftp.mozilla.org/pub/firefox/releases/78.15.0esr/mac-EME-free/en-US/ (You can change en-US to your local language code if necessary). firefox download for mac 10.10.5
Once you launch Firefox, you will notice the interface looks a few years old. That is fine. However, to make it usable on 10.10.5, you need to tweak a few settings. to make it usable on 10.10.5