emerge -av seamonkey shows that: [blocks B ] www-client/mozilla (is blocking www-client/seamonkey-1.0) This think this is very wrong. Seamonkey is new or old product, doesn't care, but this product has many changes. So many that some plugins works, some not. For clean migration from mozilla to seamonkey is usefull to have both installed. Gcc can be installed many times as you want. mozilla and firefox can be installed together. Why seamonkey is so special?
Because seamonkey doesn't work when mozilla is installed (see Bug 116231).
https://bugs.gentoo.org/show_bug.cgi?id=116231#c77 says that: "... Maybe later, some sort of slotting or whatever could be done to allow both on the same system, but for now I think it is a problem." this is time for this "later". Some issues doesn't allow to have both: seamonkey, and mozilla. Thease issues must fixed, and for this we need a bug to handle it.
No, your original complaint was "why there's this weird block, what's so special about it?" And the answer is that the block is needed, otherwise things break. If you have a patch or solution, then we are all one ear; otherwise ranting something about "very wrong" block is pretty much unproductive and useless.
The block is there as mozilla will be removed from tree once a stable seamonkey is marked in about 60 days. Package is also p.masked use at your own risk!!