In the settings under "custom add-on list". You might need to enter dev mode, which you do by tapping the Firefox logo in the about menu 5 times (you get a toast with your progress).

After that you enter the userID ND the collection name in the popup and confirm.

Details:

https://support.mozilla.org/en-US/kb/extended-add-support

And a reddit comment with a list you can use:

https://www.reddit.com/r/firefox/comments/j1ta1s/comment/g720d72/