You can download the English (US) installer directly from the Mozilla CDN. File Name: Firefox 78.15.0esr.dmg
It was the last major release to officially support Yosemite. firefox download for mac 10105 upd
How to Download and Install Firefox for Mac OS X 10.10.5 (Yosemite) You can download the English (US) installer directly
Mainstream Firefox (Versions 78+) dropped support for OS X 10.10 and 10.11 in mid-2021. The last compatible series is Firefox 78 ESR (Extended Support Release). However, a later ESR – Firefox 91 ESR – initially required macOS 10.12+. But community reports indicate Firefox 78.15.0esr is the final official build for 10.10.5. The last compatible series is Firefox 78 ESR
Modern versions of Firefox no longer support macOS 10.10. Support for this operating system officially ended on .
The Extended Support Release channel was designed to provide long-term support for older systems. While it is no longer receiving new security patches as of October 2021