Took me a while to find out how to do it. I changed the "browser.gesture.swipe.down" to close tab instead of go all the way to the bottom of the page.

1. Open a new tab in Firefox and type in "about:config" (It might warn you about changes might affect Firefox's stability, performance, etc..)
2. Search for "gesture"
3. Double click on "browser.gesture.swipe.down" and set the value to "cmd_close".

That's it.