The Release Notes are here, which describes the SeaMonkey-specific and Mozilla platform changes included with this release, as well as some of the known issues. As this is a beta version, please read carefully, before deciding if testing a beta is something you would like to do.
On a personal note, I tested SeaMonkey 2.5 Betas 3 and 4 on Windows Vista (32-bit) and on Linux i686 and x86_64. They worked well.
UserAgent: Mozilla/5.0 (X11; Linux i686; rv:9.0) Gecko/20111119 Firefox/9.0 SeaMonkey/2.6