What was happening at the time you checked this. Had you just started or shut down Firefox and it was loading/unloading? Or Firefox was open with tabs that appeared to be inactive?

This has happened on a # of occasions. I had just clicked to get into Duc kDuckGo a search engine and nothing was happening except the light was flashing indicating the hard drive was moving. I think there was an update or something because it stopped running and everything seems fine. Would like to know how I can see that an update or whatever is taking place rather than thinking there is a problem is there some way of doing that?

Firefox -- Options > Advanced -> Updates = Show Update History