MacBook Pro (W8638) shuts down unexpectedly in both Safari & Firefox

Question

My 2006 MBP has worked well. The machine model is MacBookPro 1,2; 2.16GHz, Family name: A1151, screen size 17 inch, Memory DDR2-S-667 running OS 10.6.8 Snow Leopard.
Two weeks ago it started shutting down unexpectedly. In three of the first four unexpected shutdowns I had 8-10 windows open. In three unexpected shutdowns since then I had 4-7 windows open. Most times in Safari, a couple of times when running Firefox concurrently or Firefox separately.
I've read several posts about possible battery problems. I'm on my third battery since late 2006 - which seems very fair given the amount of usage. The current battery appears to be working fine: "Charge Remaining" is 596, "Fully Charged," yes' "Capacity" 596; "Cycle Count" 211; "Voltage" 12488. "AC Charger" info: ID 0X0100, wattage 85. (As an aside, I'm on my fourth charger. Each failure was a due to a break in the plastic housing where the power cord connects.
It is important to note that the battery, on each of the shutdowns, was 75% - 100% full.
As recommended from several responsders within the Apple Support Community, I downloaded MBP SMC Firmware Update 1.5 and, I think, uploaded it to my hard drive. I say "I think" because the downloaded folder had nine icons. The one I clicked on did in fact upload to my MBP successfully - or so I thought. I've had several unexpected shutdowns since then (4) so I may have down the upload incorrectly or there may be something else wrong with the MBP. Don't know if this is helpful: I just checked "Applications" and the only firmware update I could find is under "Utilities" and it shows "MacBook Pro EFI Firmware Update.app" installed November 14, 2006. I've re-downloaded SMC Firmware Update 1.5 if that's an appropriate solution and will install 1.5 if someone would kindly advise on which of the nine icons to install.
Many thanks, Bret

Answer 1

Kappy, thanks for your help. I decided not to install MBP Firmware update 1.5 because it appears to be a fan fix for newer MBPs than my 2006.
My takeaway from your recommendation is that I shouldn't open as many windows as I have done in the past. I searched Activity Monitor and read the Support docs explanations. Also checked Console Logs (in Library/Crash Reporter) but I have no idea how to interpret on how to use that data. So I'll stick with the "fewer open windows is better" approach.
Thor