1. Open up Firefox and click on Tools > FireFTP
2. Click on the "Main" tab. Set up your FTP connection as usual.
3. Next, click on the Connection tab. The defaults are OK here. Slect the "security" dropdown and select "Auth TLS"
When you connect, you will see a welcome message from the FTP server in the log that shows at the bottom of FireFTP
It will look like this:
Congratulations, you have successfully connected to the FTP server using FTPS over TLS!
NOTE: Some versions of FireFTP seem to have problems with this method. In those versions that you cannot connect via TLS there should be an "SFTP" option. Choosing that should work for you if TLS is not working.
If you must have TLS try another client like WinSCP.