** IMPORTANT NOTE : You can only export your certificate *AFTER* Comodo (or whomever you purchased it from) issues it.
So you’ve purchased a code signing certificate from our store? Great! If you used FireFox to place the order then you’ll now need to export your certificate from the certificate store into a PFX file that you can use with the utilities that perform the code signing. This blog post will walk you through doing that.
If you ordered using Internet Explorer, click here to go to the post about exporting with IE.
To export a stored code signing certificate from FireFox to a PFX file follow these steps :
There are two ways to open the FireFox “Options” Window. The first is to just click the Tools menu. If you don’t see the Tools menu, you might need to do this :

The new default location of FireFox Options (for those that don't have a "tools" menu)
If you have the Tools menu at the top of the FireFox window, click it, then Options, then the Advanced Button (far right, top), Encryption tab, then the View Certificates button :

Firefox Code Signing Export - Step 1
Click the Your Certificates tab at the top, then Select YOUR company name (where you see K Software in the screenshot), click Backup :

Firefox Code Signing Export - Step 2 ** Depending on when you ordered, you might see "Comodo Code Signing CA 2" instead of "COMODO CA Limited"
FireFox requires a password for the PFX file.
Type in any password you like but REMEMBER WHAT YOU ENTER!

Firefox Code Signing Export - Step 3
Click OK and that’s it! FireFox will ask you where you want to save the PFX file – save it anywhere. The file is portable and can now be copied to any computer.
Additional Note : FireFox might save the file with a .p12 extension – that’s OK! Just rename the file to .pfx and you can use it with any of the code signing tools.

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Note: I followed these instructions with FF 3.6.14, but FF said “Failed to backup PKCS 12 for unknown reasons”.
Googling around led me to the solution: In my case, I had to uninstall the torbutton add-on, restart FF and it worked fine.
Hope this helps someone in the future.
I’m sure someone else will run into that so thanks for sharing!
Updated with some new screenshots and a tip about the location of the “Options” window if you don’t have the “Tools” menu.
Hi.
I have successfully signed my Binary which is an installation program. But it fails to start. It complains that the files is corrupted and recommend me to run Antivirus software. What shall I do?
BR
emwamin
Hello emwamin — it’s hard to say what might be happening there. A digital signature alone isn’t going to corrupt anything. What signing tool did you use to apply the signature?
If you right-click the EXE, go to Properties and the Digital Signatures tab, do you see the signature there?
If you are running Firefox 11 on Linux, you have to navigate from
Edit -> Advanced -> Encryption
to get to the point where you cvan View Certificates
Thanks for this post! Helped me a lot!
Best, Soenke