Compilation fails with: SkFontHost_FreeType.cpp:(.text+0x2226): undefined reference to `FT_Get_X11_Font_Format' collect2: error: ld returned 1 exit status /var/tmp/portage/portage/www-client/firefox-38.0.1/work/mozilla-release/config/rules.mk:812: recipe for target 'libxul.so' failed Upstream has already found and fixed this: https://bugzilla.mozilla.org/show_bug.cgi?id=1143411 so we just need to pick up their patch. (Yes, I've tested the patch, and it works.)
38.0.5 has been added to tree with support. Thanks for reporting.
*** Bug 551962 has been marked as a duplicate of this bug. ***