Well, I would counter that if you want to use a new application (Firefox) with the local server, there should probably be a support ticket to at least ask if they believe it should function correctly.

While you probably can assume Firefox will mostly work, there's decades of history saying you probably shouldn't, given the state of enterprise web apps. To be safe, I wouldn't assume Firefox would work any better than lynx for an internal app if neither are listed as supported (and the same goes for every browser).