If you have exported your contacts from Thunderbird to a CSV file , the quickest way to convert them into vCard format is to use a dedicated converter like
Softaken CSV to VCF Converter. It operates offline and does not require any technical skills. Simply input the tool, select your Thunderbird CSV file , preview the contacts and assign the fields (name, phone, email, company etc.) to the corresponding vCard fields and export. In the end, you will have a VCF very similar to the original CSV , able to automatically input your contact information into your iPhone, Android or iCloud or any app that supports vCard.