I think the name of the branch makes it fairly clear why both upstream (and by extention, fedora downstream) does not (and can not) include these.

What is your expectation ?

Upstream and downstream happily provided those icons until now.
Fedora also happily ships the Mozilla's trademarked Firefox icon, just not in Breeze style.
As long as Fedora ships Firefox (and not Debian's IceWeasel), I expect the proper icon to be shipped as long as Red Hat Legal does not forbid it.

firefox owns the trademark, so they certainly are free to do what they want with it (including shipping it in their source code provided to distributions).

I can only speculate why upstream once included them and now do not, that would be a good question for them.  We can only ship what they provide.

To be clear, in my prior comment, upstream refers to kde/plasma-breeze upstream.

Then revert the last update. It's clearly a regression…

My speculation: neither kde realized they do not have permission to distribute modified trademark'd icons, so removed them.  My extension, fedora (downstream) cannot freely redistribute them either.

That's why I suggested that you ask them for clarification, if you're still intersted in persuing this.

> Upstream and downstream happily provided those icons until now.

And this was a mistake, because it was illegal.

> Fedora also happily ships the Mozilla's trademarked Firefox
> icon, just not in Breeze style.

Which is allowed by the trademark and copyright license of that icon. Modifying it (and "restyling" is modification) is not.

> As long as Fedora ships Firefox (and not Debian's IceWeasel),
> I expect the proper icon to be shipped as long as Red Hat Legal
> does not forbid it.

Unfortunately the Breeze icon is not the "proper" icon in the trademark owner's view.

> Then revert the last update. It's clearly a regression…

We can't. This would reintroduce a fixed legal issue and get us yelled at by Red Hat Legal. The old versions would also have no viable upgrade path.