How about just:

'firefox url'

directly on the terminal?

yes, lol, just copy the url, find/open a terminal window, type out `firefox `, paste the url, and press enter. So much easier.

To be clear, the override script is closer to going ...

    mkdir ~/custombin/
    ln $(which firefox) ~/custombin/xdg-open
    PATH=~/custombin:$PATH slack

... than going `firefox url`