This usually happens because you chose Firefox as the program to always use for a particular action, such as opening a certain type of file or launching an application to send mail. that's why this happens..

First of all I have to admit I shouldn't have said "every time". It's most of the time. Also, exactly where would I have set this action to happen when I click a link from an email in my gmail account?

thank you

I have been having this problem for a few weeks now.

When I click a link in an email message--viewed with Outlook for Mac, latest version via MS Office subscription--two instances of Firefox open: one is blank in the BG, the other loads the link in the active window.

I have performed a FF refresh to see if that would solve the issue; it did not.

I changed my default browser to Chrome (temporarily) and the problem does not replicate for Chrome. I changed it back to FF and the problem returned.

Honestly this is a minor issue, but still one that I find a little vexing.

This only happens when FF is not already running in the BG; if it is running in the BG then a link clicked in Outlook does act as expected: it opens in a new tab.

I'm currently using FF 72.0.1 on MacOS 10.15.2.

Thanks in advance for any insights.