Are firmware updates secure?
To detect tampering, firmware updates are signed and (if you're using the recovery program that shipped with the phone) the signature is verified before install.  After a mandatory confirmation from you that the installation should start, after a reboot, the recovery program will first verify this signature and only then install the new firmware.

Note that you will always get a prompt for a ROM update. Besides allaying security concerns and giving you a bit more control over the proceedings, this gives you the opportunity to:


Make sure the battery is charged or that the phone is plugged into the mains.
Back up any data you fear may be lost should the upgrade go wrong.
Do a bit of research into the improvements in this firmware version, and decide whether it is worth your time.


So yes, firmware updates are secure and even if you are concerned, an update to the actual OS will not be installed without your confirmation.