Here's what I did:

• created a new collection and added few addons
• copied the collection id from the URL
• opened Firefox nightly and enabled custom collection menu
• input the collection id
• Firefox quits
• when I open Firefox and click on Add-ons, there were no add-ons and there was an error that "Failed to query Add-ons"

edit: added both collection id and the collection name as it appears on the URL. it is working now. Thank you for your help! :)

Well. Thanks for the info. Uninstalling

Edit: I added few add-ons to my collection.but I'm unable to add it to my Android. What could be the issue?

Kiwi supports custom extensions