Macbook Air 13" 2014 battery drain
Sadly the solution you seek is to stop using Firefox. It drains your battery much faster than using Safari. 

Close Firefox then use some Apple products and run them until your battery is very low. Then record that time. If that time is less than advertised then that is when you have a problem with the battery. 

There is no issue with your computer if your battery is drained and you are using thrid party apps.