For macOS 10.10.5 (Yosemite), the final compatible version of Firefox is Firefox 78.15.0esr. Because modern versions of Firefox require macOS 10.15 or later, you must manually download this specific "Extended Support Release" (ESR) version from Mozilla’s archives. Guide to Downloading Firefox for Mac 10.10.5
However, you can still get Firefox running by using a specific older version. Here is what you need to know to get back online safely. The Best Version for Yosemite: Firefox 78 ESR firefox download for mac 10105 upd
Alternative Browsers: If Firefox 78 does not meet your needs, some users on legacy macOS versions look toward community-maintained browsers like Pale Moon or SeaMonkey, though compatibility varies. For macOS 10
How to Download and Install Firefox for Mac OS X 10.10.5 (Yosemite) Modern Firefox releases require newer macOS versions; the