If I could reliably run a newer OS on an 8-year-old iMac, I would. But since I can't—and because the only thing I use the machine for is web browsing—the relevant question is which browser will be the safest.

Thanks for not throwing working hardware away. I imagine you would not be happy with Linux on your iMac?

I haven't used Linux since college, and I don't have enough time to get back up to speed. Good point though, and politely phrased.

El Capitan runs on mid 2007 and newer iMacs, no?

Compatible ≠ runs well. My iMac was not top-of-the-line, and I could see from how newer OSes ran on my other machines that it would be sluggish on the iMac.

Yes, Sierra is the first one that is not compatible.