Fan running in my iMac

My iMac fan runs most of the time.*It started about a week ago. *Sometimes right after I "wake" my Mac or turn it on after it being off over night, and other times it takes up to 30 minutes. (I've always slept my Mac overnight until this started happening, now I shut it down.) The fan sometimes stays on an hour, other times less than 10 minutes. *Even if I close all applications, it will still come on. *All my apps have been running slow and quirky since this started happening and I keep getting the spinning ball. *Several times I've had to force quit Firefox and Safari and one time iPhoto. *Logging into an old "temp" account makes no difference. *The fan will still come on. *I ran first aid and everything was ok. *I don't think I am using much of my memory and HD. *Under disk usage in my activity monitor it says, space free 406.14 GB. *Under Memory it says Free 571.6 MB. Under CPU it says idle 90-95%. *I'm assuming this is where the memory and HD information is dictated? I've watched my activity monitor and nothing is hogging the CPU....I assume. *When I open Firefox or Safari, for a second it may say 60%, but quickly drops to .5% or less. *However, when Firefox won't open, it shows up in red, "Firefox not responding." This started happening at the same time the fan started coming on. *It also started happening about the same time I updated Firefox. *Probably a coincidence, but worth mentioning. *I've done a SMC reset a few times but not a PRAM reset. *I'm a little scared to do that. *I was excited to read about something called iStat Pro that may help me to see my temperatures, but it's no longer available. *But I really don't think it's getting hot, because it doesn't feel hot and the fan comes on right away sometimes, even after I've had it shut down for a long time. *I've had Macs for over 15 years, but still know very little of their workings. *I do basic stuff so I have no idea how to use most applications....not even Time Machine!.....which I know I should be using. *Thanks, Terri

21" iMac, *Proc. 2.66 GHz Intel Core 2 Duo, Mem. 2 GB, Running 10.6.8*

The broken Firefox is probably causing the CPU to heat up as it works out what the problem is. I'd delete Firefox, download a new one (or switch to a different browser) and see if that eliminates the issue.

Also, and perhaps this was unintentional, but your return key appears to be broken.

Thanks. I thought of deleting Firefox, but unfortunately, I don't know how. As far as my return key, I copied and pasted the message. Maybe that has something to do with the funky stars. I didn't copy and past this time so we'll see if they are still present. I also double space after periods. Habit from my old school taught typewriter days. That could be it too. - Terri

I was thinking. That fact that the fan still comes on even when I log out and into another account, that doesn't have Firefox at all, eliminate Firefox as being the culprit? - Terri