This is still happening as of November 14 with Firefox 82.0.3. There's a mozilla thread about it that indicates that it's an elementary-OS-specific issue and that it's a regression. They've been kicking the can down the road by marking the problem as "wontfix" for each successive Firefox version rather than addressing it. If this is specifically an ...
There are many IDEs available for elementary OS. I personally use Android Studio, IntelliJ IDEA, Atom, VS Code, Scratch, and Gnome Builder. Bottomline: Whatever language you are working in or whatever your development needs are, you're covered.
They should run fine! These applications all work on Ubuntu. In general, software that works on Ubuntu also work on elementary.
I'm running Apache and PHP in a virtual server using Vagrant, and that setup works well for me. You should be able to run these applications directly on elementary, if you'd prefer.
Add the PPA by doing the following:
sudo apt install software-properties-common
sudo add-apt-repository ppa:webupd8team/java
sudo apt update
This (1) lets you install PPAs on elementary Loki, (2) adds the PPA you want, and (3) update your records. Then you can install the package you were originally after.