The internal "guts" in SeaMonkey are the same as in Firefox. The HTML rendering engine (Gecko) is the same. The Internet interface is the same. The security software is the same. If you support Firefox, you must inherently support SeaMonkey.
I downloaded uninstall_flash_player.exe from http://fpdownload.macromedia.com/get/flashplayer/current/support/uninstall_flash_player.ex e and executed it. Next, I deleted the folder at C:\Windows\SysWOW64\Macromed. Finally, I used regedit.exe to locate and delete instances of Flash and Macromed.
I then successfully installed Flash 16.0 r0 (188.8.131.525). It tested okay at http://www.adobe.com/software/flash/about/, and I was able to play the Planarity game at Planarity.net.
I removed Flash 16.0 r0 (184.108.40.2065) using "Uninstall or change a program" at Programs and Features on the Windows Control Panel. After that, I successfully installed Flash 17.0 r0 (220.127.116.11).
I previously tried to install Flash 17.0 r0 (18.104.22.168) without first removing Flash 16.0 r0 (22.214.171.1245). I then tried to do that installation after using either Programs and Features on the Windows Control Panel or uninstall_flash_player.exe to remove Flash along with deleting C:\Windows\SysWOW64\Macromed. All those attempts were unsuccessful. Thus, I think I have proven that a failure to remove a prior version of Flash before installing a new version will corrupt Windows to the extent that a manual cleanup of the Windows registry becomes necessary.