It's not a Kmeleon problem, it's a yahoo problem, which doesn't want to handle K-Meleon.
As it works, modifying the userstring, it's obvious that it's not a K-Meleon problem.
Yahoo, as some other sites, is a shit.
Mozilla/5.0 (x11; U; Linux x86_64; fr-FR; rv:38.0) Gecko/20100101 Ubuntu/12.04 K-Meleon/76.0
Mail : alain [dot] aupeix [at] wanadoo [dot] fr
Ubuntu 12.04 - Gramps 3.4.9 - Harbour 3.2.0 - Hwgui 2.20-3 - K-Meleon 76.0 rc