Not sure what you mean. Do you mean display the BCC field in the compose window by default?

Yes. I used to use the following add-on: https://addons.thunderbird.net/en-US/thunderbird/addon/use-bcc-instead-c/

After the 78 update, it is useless.

There is much jibber jabber in a bug. But it's trying to get the developers on board which may be the sticking point, so if you would like to support then please read comments and offer supprt for an auto implentation of cc and/or Bcc. https://bugzilla.mozilla.org/show_bug.cgi?id=1616514

Looks like this has been going on for a while now. Such a simple but important feature to add.

Just upgraded to 64bit Thunderbird, having been a user of 32bit for very many years. While there are no doubt many excellent improvements I have not yet found, the decision to completly destroy the perfecly functioning BCC/ CC feature was a huge error as far as I am concerned as it was something I used multiple times a day. I regret having just made my most donation as removal of BCC may force me to stop using Thunderbird completely. The "workarounds" mentioned in the threads are just not practical.

I can confirm that the developers are looking into various options to do with Bcc that are currently not part of the default setup.

the decision to completly destroy the perfecly functioning BCC/ CC feature was a huge error ... removal of BCC...

Please be respectful. Nobody destroyed anything. On the contrary, we have redesigned the whole addressing area for massive UX efficiency gains. What exactly is the issue which troubles you? Please do not create the false impression that BCC has been removed. BCC is still available, and for mouse users, it's now just one click away where before it was two. For keyboard users, it might appear a bit clumsy but there are many acceptable ways of getting BCC with keyboard, too. That said, I'm at the forefront of advocating for further UX efficiency improvements, but Rome wasn't built in a day, things can be more tricky than meets the eye and unfortunately, as Anje hinted, sometimes there's resistance from the powers that be...

Addressing an Email has more details and further links how to use the new addressing area efficiently.

Is it really that difficult to add the option to hide the "To" field?

Is there any way to have the (blank) CC and BCC fields appear by default (without having to click them) when composing an email?

re : Is it really that difficult to add the option to hide the "To" field?

As many scammers do not use the TO field when sending emails, emails sent not using it could get listed as spam. However, just because the TO field is visible, it does not mean you have to use it.

re :Is there any way to have the (blank) CC and BCC fields appear by default (without having to click them) when composing an email?

The information at the bug link provided in an earlier comment would tell you the answer is currently no. The information at the Article Link provided by thomas8 does not mention that functionality, so the answer is no. You can select the Cc, Bcc options in Account Settings, but they will not be able to appear blank. However, adding your own email address into those fields, would at least force them to appear automatically and if you do not delete your own email address it means you would get Cc'd/Bcc'd to your own emails, which is not exactly the end of the world. It does mean people will see your email address as a Cc, but since they can also see it as a 'From' there is really no problem exposing an email address that would otherwise not be hidden anyway. So there is a method, but it would not be auto blank.