Shows version 16 here. Can not help you with that.

52.5.3 ESR. It will continue to get security updates until May 2018, and you can download and install it from this page:

• https://www.mozilla.org/en-US/firefox/organizations/all/

Note : Legacy Extensions will be deleted or removed in any version update after May 2018.

You should make a backup of your Profile before going back and just because:

• https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles
• https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer

Please let us know if this solved your issue or if need further assistance.

Firefox 16 is the last version that works with OS X 10.5.

Firefox no longer works with Mac OS X 10.5:

• https://support.mozilla.org/en-US/kb/firefox-no-longer-works-mac-os-x-10-5

This probably happens because Firefox 16 doesn't support modern cipher suites and also doesn't TLS 1.2 that is required to connect to this website.

You can check the server.

• https://www.ssllabs.com/ssltest/

You can check your browser to see what your browser supports.

• https://www.ssllabs.com/ssltest/viewMyClient.html