Can you try to use this open a URL in an external Browser: https://searchfox.org/comm-central/source/mail/components/extensions/parent/ext-windows.js#341-356 That is what is used behind the scenes when WebExtensions want to open external pages.
Bug 1679849 Comment 1 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
Can you try to use this function (copy-paste it into your add-on/userscript) to open a URL in an external Browser: https://searchfox.org/comm-central/source/mail/components/extensions/parent/ext-windows.js#341-356 That is what is used behind the scenes when WebExtensions want to open external pages.