Is the fan running a problem? Most modern computers (those made in the last 15 years) have variable fans, that will speed up or slow down based on how much load the computer is under, or how much heat the computer is generating. Various programs on your computer can place more work on the computer, causing it to heat up faster and the fans to run more. It's normally not a problem so long as the fans are working properly (they are designed for this job) and the computer isn't overheating (shutting down suddenly is usually evidence that overheating is happening)

Thanks Tyler. This only happens with Firefox. Previously we were using Chrome for the work program and it wasn't a problem. Chrome stopped supporting Java so have to use Firefox. I'm using a MacPro Retina, relatively new. I had in the past used Safari and that wasn't a problem either. More annoying than anything as I need quiet. Use earplugs.

If the fan is that loud, it should be checked.

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac Options) key, and then starting Firefox. Is the problem still there?

Type about:support in the address bar and press Enter. Under the main banner, press the button; Copy Text To Clipboard.. Then in the reply box at the bottom of this page, do a right click in the box and select Paste. This will show us your system details. No Personal Information Is Collected.