May I know the number of tabs that you have opened on your Firefox browser? It could be due to the number of opened tabs that increased the CPU usage. Alternatively, it could be due to Firefox browser's parsing of heavy website elements and background processing that causes this. Try not using multiple third-party add-ons in the background as that can impact the CPU usage as well. Another solution is to use hardware acceleration on Firefox by navigating to the menu on the top-right of the browser > Settings > General > Performance > Uncheck the "Use recommended performance settings" to enable hardware acceleration.