If you are using an older Mac running , you may find that the latest version of Firefox is no longer compatible with your system. To keep browsing, you will need to download a specific legacy version designed for older operating systems. The Correct Version for OS X 10.10.5
Right-click (Ctrl+click) the Firefox app → Open → Click Open in the dialog. Or run in Terminal: firefox download for mac 10105 upd
Here is the breakdown of what version you can run, why you can’t update further, and how to stay safe. If you are using an older Mac running