I was thrilled to learn about this, too. I wasn't aware of it even as a long-time Debian user.
However, trying the specific example that was listed in the article, I installed extrepo and enabled the mozilla repo. Unfortunately, firefox is not installable on trixie in it's current form because it depends on libasound2 and the trixie package is called libasound2t64. : (
In trixie, libasound2 exists as a virtual package (ie. libasound2t64 provides libasound2) so this ought not be a problem.
It should definitely work with an 'apt install firefox' from the cli - perhaps you're using a different package management frontend? If so, installing libasound2t64 first should force it to see the virtual package - but it might be worth filing a bug report as this really shouldn't require manual intervention...
That's odd... I had no problem installing Firefox.
Wait, what! I have firefox installed through extrepo and it worked well as of yesterday.