First, install required packages:

sudo emerge --ask dev-util/icu-libs dev-util/icns2svg media-fonts/liberation \
  app-text/nghttp2 dev-libs/nspr dev-libs/nss x11-libs/libXt \
  x11-libs/libXp x11-libs/libXtst x11-libs/libXrandr x11-libs/libXinerama \
  x11-libs/libXi x11-libs/libXrender x11-libs/libXext x11-2 mesa-glx \
  media-libs/libvpx media-libs/libjpeg-turbo media-libs/libpng \
  media-libs/libwebp media-libs/libtheora media-libs/libsdl2 \
  media-libs/libpulse media-libs/libsndfile virtual/wayland-proxies \
  virtual/libX11 virtual/libXext virtual/libXtst virtual/libXrandr \
  virtual/libXi virtual/libXinerama virtual/libXrender

• **Option 1: Firefox Buildsystem**

Clone the repository:

  git clone https://github.com/mozilla/firefox-source.git
  cd firefox-source
  git checkout -b thunderbird-branch origin/thunderbird-branch

• **Option 2: Mozilla Central**

Download the source tarball from [mozilla.org](https://source.mozilla.org/).

• Configure the build:

  ./mach bootstrap

• Build Thunderbird:

  ./mach build -d -v

• Install the built package:

  sudo ./mach package -v
  sudo alpm -U thunderbird-*.pkg.tar.xz  # If using Alpine-based package manager

Run Thunderbird:

thunderbird

• Ensure you have a compatible C++ compiler (e.g., `gcc` or `clang`).
• Adjust build flags in `.mozconfig` if needed.
• Use `emerge --unmerge` to remove dependencies if switching to binary versions later.

For detailed build instructions, refer to [Mozilla's Thunderbird Build Documentation](https://developer.mozilla.org/en-US/docs/Thunderbird/Build_Thunderbird).