Starting today, Firefox will not open Adobe Acrobat files in Adobe Reader, despite my putting Reader as default in Applications tab. | Firefox Support Forum | Mozilla Support
Firefox will drop Adobe Flash support on January 26, 2021 with version 85
Enable Create PDF extension for Mozilla Firefox, Adobe Acrobat
Configure browser to use the Adobe PDF plug-in to open online PDF files
Install Missing Adobe Acrobat Extension in Firefox (Convert webpages to PDF)