Do not use browsers on outdated OS versions for sensitive financial transactions.
If a security prompt asks if you want to open an application downloaded from the internet, click . Known Limitations of Browsing on OS X 10.10.5 firefox download for mac 10105 upd
Mozilla moved users on OS X 10.9, 10.10, and 10.11 to the Extended Support Release (ESR) channel to provide a few extra years of security. However, maintaining support for decade-old operating systems became unsustainable as web technologies evolved. Mozilla Support Better Alternatives for Your Mac Do not use browsers on outdated OS versions
Because the main Firefox download button automatically packages the newest software version, you must access Mozilla's advanced storage archives to get the installer you need. Open your current web browser (such as Safari). If you do not see the Finder window
If you do not see the Finder window with the Applications folder shortcut, you can simply drag the Firefox icon from the opened disk image into your Applications folder using a separate Finder window.