How do I make Mail.app open all URLs clicked within emails to open in the background?

I use Mail.app (El Capitan OS X) and Firefox as my default mail and internet apps. I like to check my emails at once, and click on any links within the emails that I want to check out later. Every time I click on a link, though, it forces Firefox to the foreground to open the link. I'd prefer to just open all of the links in Firefox in the background while I'm still going through my emails, and then go and read the links afterwards. Basically, I want any link that I click while I'm in Mail.app to always, by default, open in the background in Firefox instead of pulling Firefox to the foreground.

I am aware that right-clicking a link and selecting "Open Link Behind Mail" will do basically what I'm asking. I am also aware that command-clicking a link will do the same thing. Basically, I'd like to make that the default behavior on all links that I click, so that I don't need to right-click or command-click. In a perfect world, that would actually apply to all links within anything in Mac (i.e. a Word document link as well).

Is there any way to achieve this? Thanks for any help!