Go to the Settings page, scroll down to the bottom.
Tap on "About Firefox".
Tap on the Firefox logo 5 times. It will display a message that says "Debug Menu enabled".
If you go back to the Settings page, you'll notice that a new option is available under the Advanced section.
It's called Custom Add-on Collections.
Tap on it, and it will ask you to enter the collection owner's ID, and the collection's name.
The ID is a unique 6-digit number that can be found in your add-on collection's URL.
The name of course refers to the collection's name, unlike the number this is something that you can choose.
For example, https://addons.mozilla.org/en-US/firefox/collections/***926754***/***ghacks***/
The ID for this collection is 926754 and the name is ghacks.
I find their argument (from 2022) about switching instances very uncompelling. Now we have instances fully dedicated to things like NSFW. It makes it a lot simpler to allow folks to block instances than adding more reasons for users to encourage defederating. In the infamous Beehaw defederation announcement they said "all we have is a nuke" and that's true. It would be nice if users could selectively disable content they don't like without having to encourage their instance admins to use that nuke.
To be clear, I'm not saying Beehaw shouldn't have defederated or that this feature could've stopped it, I'm just saying their description of defederating as the nuclear option is accurate.