Verify portage dependencies pulled by a example package, the emerge --tree option displays a dependency tree

user@host ~ % emerge -va firefox --tree

These are the packages that would be merged, in reverse order:

Calculating dependencies               ... done!                   
[ebuild     U ~] www-client/firefox-63.0::gentoo [62.0.3::gentoo] USE="eme-free hardened hwaccel system-harfbuzz system-icu system-jpeg system-libevent system-libvpx system-sqlite -bindist -clang -custom-cflags -custom-optimization -dbus -debug -geckodriver -gmp-autoupdate -jack -lto (-neon) -pulseaudio -screenshot (-selinux) -startup-notification -test -wifi" L10N="de -ach -af -an -ar -as -ast -az -bg -bn-BD -bn-IN -br -bs -ca -cak -cs -cy -da -dsb -el -en-GB -en-ZA -eo -es-AR -es-CL -es-ES -es-MX -et -eu -fa -ff -fi -fr -fy -ga -gd -gl -gn -gu -he -hi -hr -hsb -hu -hy -id -is -it -ja -ka -kab -kk -km -kn -ko -lij -lt -lv -mai -mk -ml -mr -ms -nb -nl -nn -or -pa -pl -pt-BR -pt-PT -rm -ro -ru -si -sk -sl -son -sq -sr -sv -ta -te -th -tr -uk -uz -vi -xh -zh-CN -zh-TW" 0 KiB
[ebuild  N     ]  net-libs/nodejs-8.12.0::gentoo  USE="npm snapshot ssl -debug -doc -icu -inspector -systemtap -test" CPU_FLAGS_X86="sse2" PYTHON_TARGETS="python2_7" 0 KiB
[ebuild  N     ]   net-libs/nghttp2-1.33.0:0/1.14::gentoo  USE="cxx threads xml -debug -hpack-tools -jemalloc -libressl -static-libs -test -utils" ABI_X86="(64) -32 (-x32)" 0 KiB
[ebuild  N     ]   dev-libs/libuv-1.20.0:0/1::gentoo  USE="-static-libs" ABI_X86="(64) -32 (-x32)" 0 KiB
[ebuild  N    ~]  dev-util/cbindgen-0.6.6::gentoo  USE="-debug" 0 KiB

Total: 5 packages (1 upgrade, 4 new), Size of downloads: 0 KiB

Would you like to merge these packages? [Yes/No] 

And yes, it is true, Firefox-63 started to pull nodejs as dependency on gentoo systems.

For gentoo slotted packages, like f.e. python3.6 python3.5 use the following command to find out packages depending on certain python version:

emerge -capv python:3.6

Same example showing dependency on Qt4:

user@host ~ % emerge -capv qtgui:4                                

Calculating dependencies... done!                            
  dev-qt/qtgui-4.8.7 pulled in by:                                                               
    sys-boot/unetbootin-657 requires dev-qt/qtgui:4                                              

>>> No packages selected for removal by depclean                                        
Packages installed:   1355                                 
Packages in world:    370                                                 
Packages in system:   43                                     
Required packages:    1355                      
Number to remove:     0