Firefox 49+ versions require OS X 10.9 or later.

• https://support.mozilla.org/en-US/kb/firefox-osx

You can consider to use the Firefox 45.9.0 ESR version since this version is more recent than the Firefox 48.0.2 version.

• https://blog.mozilla.org/futurereleases/2016/04/29/update-on-firefox-support-for-os-x/

All Firefox releases on the Mozilla server:

• https://download-installer.cdn.mozilla.net/pub/firefox/releases/
• https://www.mozilla.org/en-US/firefox/49.0/system-requirements/

Thanks for taking the time to send that info. I'm more confused though. Every link I click has many options. Not sure which one to download. Is there a link to the exact one I should try?

Thanks

Firefox 45.9.0 ESR:

• https://download-installer.cdn.mozilla.net/pub/firefox/releases/45.9.0esr/mac/en-US/

Firefox 48.0.2:

• https://download-installer.cdn.mozilla.net/pub/firefox/releases/48.0.2/mac/en-US/

If 48.0.2 fails to install then try 47.0.2.

• https://download-installer.cdn.mozilla.net/pub/firefox/releases/47.0.2/mac/en-US/

• https://support.mozilla.org/en-US/kb/install-older-version-of-firefox

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