Firefox and OS 10.11

It has been working just fine for me.

Try each of these in order. Test to see if the computer is working properly:

1. Reset the PRAM
2. Reset the System Management Controller (SMC)
3. Start the computer in Safe Mode, then restart normally. This is slower than a standard startup.
4. Repair the disk by booting the from the Recovery HD. Immediately after the chime hold down the Command and R keys until the Utility Menu appears. Choose Disk Utility and click on the Continue button. Select the indented (usually, Macintosh HD) volumeentry from the the side list. Click on the First Aid button in the toolbar. Wait for the Done button to appear. Quit Disk Utility and returnto the Utility Menu. Restart the computer from the Apple Menu.
5. Reinstall OS X by booting from the Recovery HD using the Command and R keys. When the Utility Menu appears select Reinstall OS X then click on the Continue button.
6. Download and Install the OS X El Capitan 10.11.4 Combo Update.

I would not reset the PRAM on a Mac that is older than four years old unless you have had a specialized PRAM battery replaced in that time frame.