I've the three open windows:
I would like to switch between them as Alt+1 - first windows, Alt+2 - second window, Alt+3 - third windows and so on.
Is there any third-party tool or other ways to do it?
I don't know if lubuntu keymapings are the same with ubuntu but the way to do it is hold down the Super/Windows Menu key and the keyboard shortcuts list will pop out. In ubuntu you can do it with Super+ 1-9.
I ended up with small script WindowsSwitcher
Now to navigate to the necessary windows it's enough to click Windows+1, Windows+2 and so on.
Ctrl+Tab is not convenient when you have more than two windows and switch pretty often between them during the coding.
Unfortuantely I didn't find in GUI of Lubuntu how to set it.
If you mean change tabs , as far as i know in firefox is :