MacBook – My Macbook Air fan is really loud

It could help if you see kernel_task soaking CPU cycles.

From https://support.apple.com/en-us/HT203184

Activity Monitor may show that a process named kernel_task is using a large percentage of your CPU, and during this time you may notice a lot of fan activity. This process helps manage temperature by making the CPU less available to processes that are using the CPU intensely. In other words, kernel_task responds to conditions that cause your CPU to become too hot. When the temperature decreases, kernel_task automatically reduces its activity.

So if your case is more than 10°F / 5°C warmer than the ambient temperature and you are seeing high CPU/kernel task usage, cooling the bottom case or increasing airflow significantly might actually speed up your Air.

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.