My computer has been slow. I get the spinning ball of death all the time it seems.

Tim, I think these were one of the best machines and I love mine, there's no reason you shouldn't get a few more years of useful life out this iMac. Are you fully up–to–date with system updates? (Does it run macOS 12.x Monterey?) Are you running a lot of things in parallel, or it's unresponsive even when you just have a few browser tabs open and not much else is going on besides that on the machine? There's definitely two separate things here that are worth verifying — one is the actual system account, as on macOS it's trivial to create a new blank user profile to test how "starting from scratch" on the Mac would feel and sign in and out of that testing account easily for comparison — you might wanna research Apple help/support resources on that topic, to try creating a new user on the side, and using that account to verify how the system and apps feel without all the history of preferences and collected data etc. of your existing user account. However it's also worth considering refreshing just your Firefox profile in case it makes any difference — the good test is trying to restart into [[Use Troubleshoot Mode in Firefox]] and keep browsing around for a bit in that mode — to help inform whether the responsiveness is impacted by any of the custom prefs, addons and similar.