Email clients and web browsers

I'm confused. For some reason, when I click a link in my email client (either Outlook or Mail), I get different 'actions' from the default browser.

For example, with Safari (not my preferred browser because I don't really like the interface) set as the default browser, clicking a link in an email opens the page in the browser, as I would expect it to.

With Microsoft Edge, the same action results in two tabs being opened in the same window, specifically my default homepage and the page linked in the email.

With Firefox (my preferred browser - until now) clicking the same link opens two separate windows, one with default homepage and another with the page linked in the email.

Does anyone have any ideas why this should be the case, please, and is there a way to only open the linked page, not the homepage?

Thanks

EDIT: This only happens when the browser is not already 'active', i.e. running either in the background or with an open window.