I uninstalled Adobe Acrobat Reader DC. I went to Adobe's website and downloaded a new installer and reinstalled.

I checked for updates within Adobe Acrobat Reader DC. It installed an update, and I am back to Adobe Acrobat Reader DC 15.009.20077 on both of my computers. Firefox once again says: outdated 15.9.20069.28170

This is a known issue with the update checker. If the programs web site says you are up to date, then you are fine. Don’t worry.

If you can't update the Adobe Reader program from within the reader (i.e. you have the latest version) then you can ignore the report on the plugin check page. There have been problems in the past with detecting the latest version because Adobe changed the way the AR reports itself. Adobe doesn't offer the latest version of the Adobe Reader on its downloads page and the only way to be sure that you have the latest AR version is by checking for updates in the AR program until no updates are available.

• bug 1214838 - Adobe Reader DC 15.009.20069 is showing as outdated, needs to add entry

Please do not comment in bug reports
https://bugzilla.mozilla.org/page.cgi?id=etiquette.html