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How to Install and Use an S/MIME Certificate in Mozilla Thunderbird? 

 
Overview 
This guide provides the step-by-step process for installing an S/MIME certificate in Mozilla Thunderbird and enabling secure email signing and encryption. This helps ensure the authenticity and confidentiality of your email communication. 

 
Prerequisites 
Before you begin, ensure you have the following: 

Procedure 

 
Step 1: Download Your S/MIME Certificate 
 
Download the PKCS#12 file (.p12/.pfx)* from your Certificate Authority account. You will be prompted to create a password when downloading the file. Keep this password secure. 

*: To submit a CSR and create a pfx file please refer to the below document: 
How to submit a CSR and Export a Certificate in PKCS12 Format on macOS? | Sectigo® Official 

 
Step 2: Import the S/MIME Certificate into Thunderbird 
1. Open Mozilla Thunderbird. 

 
2. Go to Account Settings. 

 
3. Click the End-To-End Encryption tab. 

 
4. Scroll down to the S/MIME section and click the 'Manage S/MIME Certificates' button.  

 
5. Under 'Your Certificates', click the 'Import' button. 

 
6. Locate and select your. p12/.pfx file. 

 
7. Enter the certificate password when prompted.  

 
8. Verify that the certificate now appears in the Certificate Manager. 
 
Step 3: Assign the Certificate to Your Email Account 
 
1. Stay in Account Settings. 
2. Under the S/MIME section, click the 'Select…' button. 

 
3. Choose your installed certificate for both digital signing and encryption.  

 
4. Click OK to apply the settings. 
 
Step 4: Send Signed or Encrypted Emails 
 
- To digitally sign an email: Open a new message → Security → Digitally Sign This Message. 

 
- To encrypt an email: You must have received a digitally signed email from the recipient to obtain their public certificate. Then open a new message → Security → Encrypt This Message. 

 
 
Conclusion 
After completing these steps, you can securely send digitally signed and encrypted S/MIME emails through Mozilla Thunderbird. 
 
Verification 

To confirm successful setup: 

 
 
 
Related Articles:  How to submit a CSR and Export a Certificate in PKCS12 Format on macOS? | Sectigo® Official
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