Loathe though I am to respond to a bot, the button is set in Roccat Swarm to Universal Scrolling. If I disable it, it disables it also in Windows. There are some functions in Swarm to have it open a folder or program or window but not specifically to open a link in a new tab in Firefox, sadly. And yet for 95% of the time I'm using it, that's exactly what it does.
Edit - I tell a lie, there is a function for that, but it's not ideal for me as it directly switches to the new tab as it opens instead of opening the tab in the background and allowing me to continue to browse. I'm being picky, I know.
Another Edit - It's happening right now in YouTube, yet if I use it to click on a link on any other random website, it opens them in the background as it should. So it's relegated entirely to YouTube, it seems.