What is firmware.zip?
Firmware.zip is a part of each OTA update from HTC. It contains important part of the firmware like radio, bootloader, recovery, kernel etc..
Why it's important to flash firmware.zip and when should I flash it?
It is recommended to have flashed latest firmware on your device for best modem performance and system stability. Sometimes it's necessary to have latest bootloader flashed because otherwise your device might not work well with new system update.
Does firmware.zip contains radio?
Yes, it does contain radio, and for S-ON devices the only method to update radio is to flash latest firmware.zip
Why it's important to flash it before flashing custom ROM?
Because firmware.zip contains boot.img (ramdisk and kernel) that will replace custom boot.img from any custom ROM you have. That's why you have to flash firmware.zip first, because otherwise boot.img will be overwritten with stock one.
If you already flashed firmware.zip after flashing custom ROM, just flash this particular custom ROM again.
Does firmware.zip contains recovery?
Yes, it does contain recovery image, that's why after flashing firmware.zip you will have to flash custom recovery again, to be able to flash custom ROM afterwards.
Can I flash this firmware.zip on any HTC One?
Not if your device is S-ON. To be able to flash it on S-ON device, your modelid (example: PN0710000) and cidnum (example: HTC__001) must match the firmware.zip version you are about to flash.
If my modelid or cidnum doesn't match, can I brick my device if I try to flash firmware.zip anyway?
As long as your device is S-ON flashing firmware.zip with non matching modelid or cidnum will result in error and nothing will be flashed, so there is no risk of brick.
Do I need to have locked, unlocked or relocked bootloader to be able to flash firmware.zip?
Your bootloader needs to be relocked.
If I can't flash newest firmware.zip because of wrong modeid (MID) or cidnum (CID) does it mean I can't use custom ROM based on the latest HTC update?
In most cases you should be able to run any custom ROM as long as hboot update is not mandatory.
What if there is no firmware.zip for my MID/CID?
Try to find latest OTA update for your original stock ROM and send it to me. I will post firmware.zip here.