MBP running like sludge...?

Hey,

I have a MBP:
v10.5.8
2.33GHz Intel Core 2 Duo
3GB 667 MHz DDR2 SDRAM
500GB drive with 140GB unused.

However, lately my MBP makes a Commodore 64 look like a Cray.

I just booted Activity Monitor and Firefox CPU usage is bouncing anywhere from 8.0 to 91.3, which might be the culprit, but I'm running way less tabs in Firefox today than I have in the past and in the past my MBP was lightning fast.
Other than that, I don't have a lot going on (as compared to what I've run concurrently in the past...I remember in the past running iDVD and iMovie, iTunes, Word, etc., etc, all at the same time. and my MBP was loving it. Today, I'm running Firefox, Safari and iTunes and you'd think that somebody shot my computer full of heroin...I've never seen it so groggy and unresponsive. I'm getting a lot of momentary hangs and rainbow pinwheels for very simple tasks like Back browser button and switching tabs).
My computer seems to be doing a lot more thinking and a lot less doing.
Even typing this post out, sometimes the cursor will stop and I'll be 5-6 words ahead of it by the time it finally catches up to what I've just typed.

Any suggestions as to what I should look for or what might be causing this kind of borderline coma? Any troubleshooting tips?

Do I need a program like Spring Cleaning or something like that?
I run Onyx pretty frequently, so if anybody has both Onyx and Spring Cleaning, please let me know if you think SC is worth getting (since I already have Onyx).

Any help/suggestions greatly appreciated.

Thanks,

J.

 

Yeah, sometimes Firefox bounces between 8.0 (idle) and 91.3 (multiple tabs open). Any reason Firefox is so voracious?

 

How do I do that?