It could be a variety of things to be honest. Firefox has the ability to use various extensions and these sometimes update/work in the background or show some sort of activity depending on their purpose. Disabling those and having another go as suggested would be a good starting point. It could also be related to the way a website has been programmed to work, so if you can provide an example of a site where you encounter this I would be happy to test this on my end (Firefox user on both Win and Mac). If you encounter it after a reboot, it could even be related to Windows still adjusting itself in the background although that is unlikely.
Worst case it is related to something you do not want on your system. Have you had a go at a malware/antivirus scan to make sure it is not something from within that category?