Yasm is a cross-platform assembler. It is used by the Mozilla codebase to generate routines for decoding the WebM video format and JPEG images.
Mozilla currently requires Yasm 1.1.0 or newer.
Obtaining Yasm
Windows
Yasm is provided in MozillaBuild 1.5.1.
Mac
You can build Yasm from source by downloading the package here. You'll need version 1.1.0 or newer.
Yasm is also available through HomeBrew and MacPorts. Install it by running
brew install yasm # or sudo port install yasm
If you install using MacPorts, make sure you have version 1.1.0 or newer by running yasm --version. You can run
sudo port selfupdate && sudo port upgrade yasm
to update to the newest version.
GNU/Linux
Yasm is available through most GNU/Linux distributions' package managers. However, you'll need to ensure that your package manager provides at least Yasm 1.1.0; older versions may not work properly.
On Fedora (<22)/Redhat systems, run
sudo yum install yasm
On Fedora 22
sudo dnf install yasm
On Ubuntu/Debian systems, run
sudo apt-get install yasm
If a new enough version isn't available through your package manager, you can either build Yasm from source or disable the modules in Mozilla which rely on Yasm.
Building without Yasm
If for some reason you cannot install Yasm, you'll need to disable the WebM decoder and the optimized JPEG decoder. Do this by adding the following lines to your mozconfig:
ac_add_options --disable-webm ac_add_options --disable-libjpeg-turbo