It's not really a full fork, but I'm fairly confident that Iceweasel, the patched and no-branding Firefox that ships with Debian, will not have this problem.

(So as I Debian user I don't really care, but it worries me slightly for the future of Mozilla.)

Mozilla has said that this requirement is set by one flag at build time, so building a version without this requirement should not be any more difficult than just compiling Firefox I have never done that, but I think it's slightly non-trivial. The hardest part though would be distributing the fork though, so a Linux distribution like Debian mentioned this change as others have mentioned would be one way to build a popular fork without this requirement.

It's nontrivial, but others have worked out the details:

[1]: https://git.exherbo.org/desktop.git/tree/packages/net-www/fi...

[2]: https://git.exherbo.org/desktop.git/tree/exlibs/mozilla.exli...

you don't need to fork. There are four versions without requirement: nightly, dev edition, unbranded stable and unbranded beta. What more do you want?

Difficult how? Even though Mozilla is going to providing builds of just such a fork themselves? Is it particularly hard to build firefox?

How about Pale Moon ?

It's trivial.