Firefox i my default browser. When I click on a hyperlink in an e-mail two Firefox windows open at once, not one.
This issue can be caused by an extension that isn't working properly.
You can try to disable all extension to see if it still happens.
If it works in Safe mode or with all extensions (Tools > Add-ons > Extensions) disabled then try to find which is causing it by enabling one extension at a time until the problem reappears.
Close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")
You can also check for issues caused by DDE.