I think you're misunderstanding trademarks. As an example: if I buy a 12 pack of Coke, I can set up a table and sell them one can at a time. Trademarks require that I don't present that I'm sponsored by Coke, or label my non-Coke as Coke, or a variety of other things that would confuse consumers. That's the entire premise of trademarks: avoiding consumer confusion.
To your example: Debian had to rename their package because they were making modifications to the code of Firefox, so it wasn't Firefox anymore.
OBS could try to make the claim that how Fedora is packaging their code modifies the code, but that's way more tenuous given that building the product with different versions of its dependencies isn't really changing the code.
I think it's easy enough, ignoring the technical details of why, to just look at the results: the version of the app in fedora's repos is inferior to that on flathub produced by the developers. Exactly why this is the case is irrelevant, what matters is that users are getting confused, thinking they are getting an official version of the product, and that this is causing damage to the developers who own the trademark (extra effort in dealing with bug reports, loss of reputation).
Considering that trademarks have a long case law with being used to force redistributors offline (i.e. to force my app to not be available on Download.com); I think it is fair to say that it is well established that a trademark gives the owner total distribution control except where the first-sale doctrine intervenes (resale of a product acquired through a legal seller). That doctrine only exists for physical products though, and specifically for items that can be “bought.”
In the real world, Coca-Cola would "force a redistributor offline" by not selling them products, and setting up agreements with their distributors to limit who they can sell to. That's not trademark, and it doesn't transfer to free software.
To your example: Debian had to rename their package because they were making modifications to the code of Firefox, so it wasn't Firefox anymore.
OBS could try to make the claim that how Fedora is packaging their code modifies the code, but that's way more tenuous given that building the product with different versions of its dependencies isn't really changing the code.