To use JOSM to edit fosm content you will need to make the following configuration change:
- Within JOSM go to Preferences (from the menu Select Edit/Preferences or press F12).
- Select the Connection Settings tab, which is the second one down.
- Change the Base Server URL from http://www.openstreetmap.org/api to http://fosm.org/api
- Check the "Use Basic Authentication" choice (FOSM doesn't support OAuth).
- Change the username and password to those of your fosm account.