Firstly you need to enable the bookmarks toolbar.
To turn on the Menu bar or 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 "
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!