Does a firmware update for dell optiplex 755 optical drive fix intermittent rebooting issue

You can try to attach the drive with a different USB cable to another computer. That way you will rule out connectivity issues and drivers as a possible cause.

In case the issue persists, there is a good chance that something wrong with the enclosure or the HDD itself. In both cases, you can:
-RMA the drive if under warranty.

There is no need to "disable" firmware updates.
The computer will only accept "valid" updates.
So, how would somebody be able to access your computer, in order to try to update the firmware. Prevent access to your computer, and you will have prevented the firmware from being updated.

Find your specific model on http://support.dell.com
First, look for a "chipset" device-driver. If it exists, install it first, and reboot.
Second, look for an Ethernet device-driver. Download it to a USB memory-stick, and move the memory-stick to the Dell, and install it.
Third, the order of the other device-drivers does not matter, but having a working Ethernet connection makes it much easier to download/install the other device-drivers, and also to run "Windows Update" to download more device-drivers directly from Microsoft's web-site.

http://www.dell.com/support/drivers/us/en/04/driverdetails?driverid=2DWTY

what model is this small form factor ? mini tower?anyhow, here are the things that you may consider .
if its a small form factor unit then you may need to get a video card that is Low Profile
and also consider the power supply :
The ATI Radeon HD 4650 512MB Low Profile PCIe x16 graphics card should work fine with the 275w PSU.
Max power draw is 48 watts

Motherboards are about $50
http://shop.ebay.com/?_from=R40&_trksid=p3907.m38.l1313&_nkw=optiplex+755+motherboard&_sacat=See-All-Categories
replacement instructions are here
http://support.dell.com/support/edocs/systems/op755/en/UG/HTML/index.htm

if you have dell datasafe installed in pc, kindly update to latest version of dell datasafe.

if no dell datasafe installed, upgrade firmware for your optical drive from suppot.dell.com

download and run piotodma.exe from support.dell.com

make sure that you are closing the CD and also make sure that the cat isnt licking the dogs balls while your doing it.

Sounds like you only have power going to your motherboard.

The power supply might be bad on it, that or maybe the power cord is going bad and is only letting a small amount go through. I don't see why there should be a problem if you have even a little bit of power going though.

Try the fix listed here - the Fix It image.
http://support.microsoft.com/kb/314060