Email client links opening two Firefox windows

I am using Firefox 148.0 and Mac Tahoe 26.3.1. When I click on a link in an email, two Firefox windows open. The window to the link opens, along with my homepage window, so two windows are opening in Firefox instead of just the one containing the link address. This is happening when using Thunderbird. However, I have tested this using Apple Mail and Outlook, and the same problem occurs. This problem happens whether I have Firefox already open or not. I have opened the email links with Firefox running in Troubleshoot / Safe mode and the problem persists. I have also cleared Firefox cache / site data / history and the problem concerning email links is still there. This problem arose after installing Firefox 148.0. Anyone able to help solve this problem? Thank you for taking the time to read this report.