Since this is an older build, macOS might block it due to an expired gatekeeper ticket. If you see the warning “Firefox can’t be opened because it was not downloaded from the App Store” :
Using a browser that is "too new" will result in the app crashing instantly upon opening, while one that is "too old" will not render modern websites correctly. 2. Where to Download Updated Firefox for Mac 10.10.5 firefox download for mac 10105 upd
For users on legacy operating systems like Yosemite, Mozilla provides a special branch called (Extended Support Release). Unlike the standard consumer version that updates every four weeks, ESR focuses on stability and security patches for enterprise users—and incidentally, for old Macs. Since this is an older build, macOS might
For Mac users running macOS 10.15 Catalina, Firefox remains a top-tier web browser that delivers speed, privacy, security, and an extensive library of customization options. The installation process is straightforward and can be completed in just a few minutes. Best of all, you can install the most up-to-date version of Firefox directly from the official Mozilla website without having to search for outdated releases. Where to Download Updated Firefox for Mac 10
Important note