It would seem you have set Google Chrome as default browser and also set http to open using Google Chrome. then you've selected to use Google Chrome is thunderbird as per your supplied image. So links should open in Google Chrome. When you click on a hyperlink in Thunderbird, it follows whatever settings are set up on your computer. Have you set Firefox as your default Browser ? Have you set up all the various protocols etc on computer to use Firefox. Is Firefox an allowed program in your Firewall ? In the Thunderbird options > Attachments - you supplied an image showing you had selected Google chrome. Is there a reason you have not selected Firefox assuming you have got all your computer settings correct? Did you open Firefox browser and check it is set as default in Options ? Did you set Program Default and associations to use firefox? Did you check your Program access and default custom settings? Did you open Google Chrome and allow it to set itself up as default ?