Firefox got an xforms use flag via bug 147107. Seamonkey 1.1 should as well. Attached is a patch for seamonkey-1.1.ebuild that adds it akin to the firefox ebuild.
Created attachment 108671 [details, diff] Patch to seamonkey-1.1 ebuild to add xorms USE flag I'm using seamonkey compiled w/ xforms support to post this; it works as expected at least on x86.
Fixed, thanks.