Hi You can read about this [http://mzl.la/1xKrDRk here]. What you want is [https://ftp.mozilla.org/pub/firefox/releases/51.0.1/win64/en-US/Firefox%20Setup%2051.0.1.exe here].

You would probably do better to continue using the latest Firefox release and update that as necessary. It should now be Fx54. That is then safe secure and supported. As a Windows user the simplest option for installing an older version of Firefox is to install the third party Firefox portable. That can be downloaded on to your Hard Disk Drive or even on to a memory stick to use on other Windows computers. *The portable Fx45.'''9 ESR''' is the one you would appear to need. <br> https://portableapps.com/apps/internet/firefox_portable/legacy <br>(Version 45 (ESR) - Security Patches Through June 13, 2017) Of course if the Webex meetings need to be safe and secure you could have problems with using outdated software.