firefox should be buildable without spellcheck support and thus without dependence on hunspell or myspell (why the ridiculous double anyway?)
Apparently FF misses a configure option to not compile against a spellchecker, that's why it's been made a dependency. Perhaps someone is able to provide a patch? Close if you consider upstream or invalid.
You can file a bug upstream, if they decide they want to reintroduce a useflag we will be willing to backport to esr branch.