For Firefox:

https://github.com/mozilla/video-bg-play

This works great, thank you.

For others: Settings > Media > Enable Background Audio

I don't know why your comment is downvoted because I use this feature of Brave very often and I also exclusively watch YT in Brave mobile (no ads).

There are a lot of people that don't like Brave's business model. But I've never given Brave a dime and turn off their ad network stuff and they've saved me hundreds of dollars on Youtube Premium over the years.

For me, it was as easy as adding a shortcut to the YouTube homepage on Brave that it basically acts like the YouTube app, but with ad blocking built in. It's the only way I watch YT videos on mobile.

You might be interested in GrayJay app.

It's a really cool idea for an app. Pity that Google takes down the videos where Rossman talks about it.

New Pipe X if you need a full client does that as well.

I know of NewPipe (Android) but can't find anything about New Pipe X