Created attachment 413554 [details] build.log On amd64-fbsd, www-client/firefox-41.0-r1 fails to build when dev-lang/yasm is not installed. configure: error: yasm is a required build tool for this architecture when webm is enabled. You may either install yasm or --disable-webm (which disables the WebM video format). See https://developer.mozilla.org/en/YASM for more details.
I've added dev-lang/yasm as a dep starting with firefox-42 for {amd64,x86}{,-fbsd}.