after having re-emerged firefox (64 bits), the problem remains the same. I tried to use the binary version for a while and it seems that the odd behaviour is away. (sorry for my poor english
I wonder if that comes from Gnash still experimental include in firefox 64. Anyway, that doesn't matter, I will be using firefox-bin until the 3.2 release will be out and see if the problem keeps going on.
@ lxnay and cvill64 : thank you very much for the good job done for the noobs to gentoo like me and for having build a gentoo system oriented for people new to linux (and for those who don't want to spend their time to install and compile all the stuff from scratch ! )
End of November will be interesting ...