Hello fbo20 , check it in Safe Mode, in Safe Mode window select "Start in Safe Mode" and see if this happen again, if not then see: Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

(If the problem does not happen in Safe Mode, it is most likely because of an extension or maybe a theme).

thank you

I really dont understand what's happend here, now my windows are working normal, but earlier today it doesn't work, i've try not once or twice, but at least 20 times, and don't work, but now is working again, but until when? who knows...

It still doesn't working i cannot open a second firefox window i've try the shotcut Ctrl + N, but not happens, i've already gone to menu too menu>new tab>new window (Ctrl + N) but doesn't works too, i'm thinking about go back to firefox 18 or older, because without second window firefox 19.0.2 its not good enought :/

Fbo20, quando você abre com os addons desabilitados (como descrito pelo ideato) a opção de nova janela funciona?

Instruções em português: https://support.mozilla.org/pt-BR/kb/Modo%20de%20seguran%C3%A7a

entao é o que eu falei, ele funciona, mas depois volta a nao abrir... deve ter sido algum bug do 19.0.0, eu reinstalei o firefox com os complementos dele e agora parece que ta tudo certo... agradeço a todos que me ajudaram...