: Firefox 52 ESR is the last official version that supports NPAPI plugins (Java, Silverlight, Acrobat).
Downloading and using old versions of Firefox is on the modern web. These versions: Legacy Add-on Support on Firefox ESR - The Mozilla Blog Download Firefox 12-51 52 Esr
Once installed, Firefox will immediately try to update itself to the newest version. To prevent this, go to Options/Preferences > General > Firefox Updates : Firefox 52 ESR is the last official