User blog comment:Sannse/Even-faster-Monaco/@comment-1002946-20100317142032

That's because as of now, SeaMonkey hasn't received a stable update to upgrade it to Gecko 1.9.2+ and thus it is unable to render the "-moz-linear-gradient" line among others, alike to Firefox 3.5 and lower.

The beta SeaMonkey 2.1 seems to support Gecko up to the experimental 1.9.3 branch, which puts it equal to or above to Firefox 3.6's capabilities and should be able to display gradients correctly.