Apps reading net drives is freezing them

I've a NAS and its folders I need mounted automatically using /etc/fstab file.

Everything works like a cham, but when I use some GTK apps like Firefox and it tries to look for a folder, it seems the app looks into the NAS folders and it takes a lot of time, freezing the app. It happens the first time the net drives are accessed for the first time I boot.

Whay does it happen? How could I solve it? It's very annoying.

Thanks!