Will this work to hook up printer to laptop? my ethernet out is used for cable modem connection.

No! This is not a Ethernet cable. While the one end of the plug is similar to Ethernet, it's not the same. It's really a USB cable with a similar plug to Ethernet that plugs into a APC USB device. Why APC went with this type of connection on some of their UPS (Uninterruptible Power Supply) units I don't know. How you came across this cable, I don't know. You sure can't just Plug a Ethernet device directly into a USB port. It's not remotely close.Your best bet is to get a Router. This will create your own Private Network. You would then plug your cable modem into the Router, and then you can plug your Ethernet Printer into one of the Ethernet ports on the back of the Router, and your Laptop in the Back of the Router for a wired connection. If your laptop supports Wifi, and you get a Wifi capable Router, You don't need to plug your Laptop into the Router, you can be wireless and move around your place. You leave your Router, Cable Modem and Printer at the current location plugged in. You'll still be able to print to your printer even wireless. You don't need to spend a bunch of money on a Wifi Router. If you don't need wireless, a cheaper wired only router will work just fine also. What a Router does is take that 1 IP address your ISP (Comcast, AT&T, whoever) gives you and your Cable Modem, something like 107.25.254.55 and creates a Private Network for you and your different hardware. This allows more then 1 Computer to access your Internet. Generally your devices will now have a IP of 192.168.0.1, and 192.168.0.2, 192.168.0.3 and so on. This is one of the Reserved IP address used for Routers. 10.0.0.1 is another, but 192 is far more popular. So your PC would be 192.168.0.2 because generally the Router will assign it's self as 192.168.0.1. Your Printer may end up 192.168.0.3. If you go Wifi with the Router, now your Smartphone or tablet can also connect to your Cable connection and they'll also be assigned their own IP number on your private Network. It opens the world up for you. Then yo can just get some normal Ethernet Patch cables. They'll have the same plug on both ends. Any other questions just ask!

It suppose to be 10 pin RJ50, not 8 pin RJ45 connector.

It's an RJ 45 on one end but with only two pair of wires (4 total) inside, not the usual 4 pair Ethernet - the other end is regular USB connector to connect to computer. The APC's before this used a Serial port connecter to the computer with an RJ 45 to the UPC unit. Double check your product documentation to make sure you get the right one for your unit - you can download the docs at APC .com

How long is this cable? I need longer than 6 ft. Maybe I just need an extention rj 45 cable?

if you search for RJ45 cables by the length you need (i.e. 15 foot RJ45 cable in electronics), you'll find what you want

Can i use this to upgrade the firmware on my apc smt 750?

No, for additional options like firmware upgrades you would need the 940-0024 series of smart signaling cables. This is a simple signaling cable and has limited access to the UPS.

Brand	APC
Product Dimensions	14.73 x 1.02 x 10.16 cm; 9.07 g
Item model number	AP9827
Manufacturer	APC by Schneider Electric
Series	AP9827
Color	Black
Processor Count	1
Computer Memory Type	DDR3 SDRAM
Hardware Platform	PC
Are Batteries Included	No
Item Weight	9.07 g

APC AP9827 Simple Signalling USB to RJ45 UPS Cable

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