Windows may warn you that it could not verify the publisher when you try to run the installer. This is because the SeaMonkey installer for Windows is not digitally signed. As long as you downloaded the installer from either www.seamonkey-project.org or mozilla.org, you can safely ignore it (more experienced users may use the checksums linked to from the download pages to verify the integrity of the installer).
Windows 7 and above will by default present you with an additional warning after extracting finished. You also need to confirm this User Account Control warning in order to proceed with the installation.

To install SeaMonkey by downloading the SeaMonkey installer, follow these steps:

1. Click the Download link in the relevant Windows column for your architecture (x64 for 64-bit or x86 for 32-bit) on the site you're downloading SeaMonkey from to download the installer file (seamonkey-2.53.1.en-US.win64.installer.exe or similar file name) to your machine.
2. Navigate to where you downloaded the file and double-click the installer file icon on your machine to begin the Setup program.
3. Follow the on-screen instructions in the setup program (see the note about Unknown Publisher warnings above). The setup program gives you the option to launch SeaMonkey once the installation is completed.

To install SeaMonkey by downloading the .zip file and installing manually, follow these steps (normal users should NOT do that but use the installer as described above):

1. Go to a Mozilla FTP server or one of its mirrors, navigate to the appropriate SeaMonkey releases directory and the subdirectory for your language in its win32 or win64 subdirectory. Click the link to seamonkey-2.53.1.en-US.win64.zip (or similar file name) in there to download the .zip file to your machine.
2. Navigate to where you downloaded the file and double-click the compressed file.
3. Extract the .zip file to a directory such as C:\Program Files (x86)\SeaMonkey or C:\Program Files\SeaMonkey.
4. To start SeaMonkey, navigate to the directory you extracted SeaMonkey to and double-click the seamonkey.exe icon.

To install SeaMonkey, follow these steps:

1. Click the Download link in the macOS column on the site you're downloading SeaMonkey from to download the disk image file (seamonkey-2.53.1.en-US.mac.dmg or similar file name) to your machine.
2. Double-click on the .dmg file. The disk image will now mount and a new Finder window should appear.
3. Drag the SeaMonkey icon to the Applications folder.
4. Eject the disk image.
5. If you want SeaMonkey to be accessible from the Dock, simply drag the SeaMonkey icon from the Applications folder to the Dock.

There is no installer available, but just extracting the tar.bz2 to the default directory (usually /usr/local/seamonkey) achieves the same result as an installer would.

To install SeaMonkey by downloading the tar.bz2 file:

1. Create a directory named "seamonkey2" (mkdir seamonkey2) and change to that directory (cd seamonkey2).
2. Click the Download link in the relevant Linux column for your architecture (x64 for 64-bit or x86 for 32-bit) on the site you're downloading SeaMonkey from to download the package (seamonkey-2.53.1.en-US.linux-x86_64.tar.bz2 or similar name) file into the seamonkey2 directory.
3. Decompress the file with the following command:

   tar jxvf seamonkey-2.*.tar.bz2

   This creates a "seamonkey" directory under your seamonkey2 directory.
4. Change to the seamonkey directory (cd seamonkey).
5. Run SeaMonkey with the following command:

   ./seamonkey

To hook up SeaMonkey complete with icon to the GNOME Panel, follow these steps:

1. Click the GNOME Main Menu button, open the Panel menu, and then open the Add to Panel submenu and choose Launcher.
2. Right-click the icon for SeaMonkey on the Panel and enter the following command:

   directory_name/seamonkey

   where directory_name is the name of the directory you extracted SeaMonkey into. For example, the default directory that SeaMonkey suggests is /usr/local/seamonkey so the full command would be /usr/local/seamonkey/seamonkey.
3. Type in a name for the icon, and type in a comment if you wish.
4. Click the icon button and type in the following as the icon's location:

   directory_name/chrome/icons/default/default.png

   where, again, directory_name is the directory where you installed SeaMonkey. For example, using the default directory /usr/local/seamonkey results in an icon location of /usr/local/seamonkey/chrome/icons/default/default.png.

The procedure is quite similar in the Xfce environment:

1. Right-click the Applications Menu button, open the Panel menu, then choose Add New Items.
2. Select Launcher and click Add, then close the Add New Items dialog.
3. Right-click the new launcher (the one with no icon at the far right of the panel), then select Properties to open the Launcher dialog.
4. Click the document icon with tool-tip "Add a new empty item" and fill in the Name and Command fields in the Create Launcher dialog.
   Name: SeaMonkey
   Command: directory_name/seamonkey (e.g. /usr/local/seamonkey/seamonkey)
5. Click the "No icon" button and navigate to directory_name/chrome/icons/default/default.png. Press OK, then Create, then Close.