The link in my signature will provide a good safe version 7.11 for firefox which was just released yesterday afternoon.
If you don't mind typing commands or copying and pasting into a terminal.
I don't program so I don't use the JDK nor do I have it installed.
The instructions for installing in the Manually update java link have not even been updated from 10 to 11 so you'll have to remember to change that when copying and pasting.
cavsfan@cavsfan-MS-7529:~$ java -version
java version "1.7.0_11"
Java(TM) SE Runtime Environment (build 1.7.0_11-b21)
Java HotSpot(TM) 64-Bit Server VM (build 23.6-b04, mixed mode)