In order to avoid possible rendering errors, you shouldn't open .pdf documents directly in Firefox, but in Adobe Reader®.
You need to change Firefox preferences in order to automatically open .pdf files externally.
Open Firefox and click on the menu symbol 
in the upper right corner. In this menu, choose preferences ("Einstellungen").
Click on applications ("Anwendungen") and change the drop down menu for "Portable Document Format (PDF)" to "open with Adobe Reader DC" ("Mit Adobe Reader DC öffnen") (refer figure "preferences in Firefox").
Your preferences will be saved automatically.
From now on, every .pdf file should be opened with Adobe reader automatically (when using Firefox).