Firstly you need to enable the bookmarks toolbar.
See here: https://support.mozilla.org/en-US/kb/customize-firefox-controls-buttons-and-toolbars and scroll down (near the bottom) to find the section "To turn on the Menu bar or Bookmarks toolbar"
Once the Bookmarks toolbar is enabled, right mouse click anywhere (where there's empty space) within the new toolbar and choose "New Bookmark". Give it a name, and in the location field paste the JS code from here. Click add. It should show up as the name you have chosen with the default icon of a globe. Then follow the instructions in this post to use the tool.
Cheers.
Cheers! That works! Now just to find out how many groups I can join. I read somewhere that the limit was 75 only according to something I found here https://www.soundcloudcommunity.com/soundcloud/topics/i-get-suspended-from-time-to-time-for-posting-to-too-many-groups-how-do-i-know-the-limit-is
If that is the case then this might not even really be needed. Will still come in handy though cheers!