I can not find any suggestions anywhere in the web that you can set it to a null value. That would set it back to default and remove the entry from the cofing.js file would be the normal arrangement for a string that does not appear in the about config unless it is modified.

From my reading in the last few minutes I appears the option to to override the default string with another, not suppress it.

But then why would you want to randomize the user agent string. You send me an email and your mother an email. What possible value is randomizing the MUA string going to have.

I can not find any suggestions anywhere in the web that you can set it to a null value. That would set it back to default and remove the entry from the cofing.js file would be the normal arrangement for a string that does not appear in the about config unless it is modified. From my reading in the last few minutes I appears the option to to override the default string with another, not suppress it. But then why would you want to randomize the user agent string. You send me an email and your mother an email. What possible value is randomizing the MUA string going to have.

I prefer to hide real user agent because it contain my OS version, software version etc. for example User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:78.0) Gecko/20100101 Firefox/78.0

I concern on privacy. I want to send the message on email not my software/os version on email.

well why not change it to User-Agent: I want to hide this

it is your setting it to nothing that is resulting in your change not being saved.

just reset privacy.resistFingerprinting to default fixed the issue. now thunderbird respect general.useragent.override