When I check the Command Line Options article, it indicates that the "-remote" switch was removed several versions ago. Is "--remote" a system-level switch?

https://developer.mozilla.org/docs/Mozilla/Command_Line_Options#Remote_Control

I didn't saw that. So how can I open now from console 3 separate windows with URLs?

When you say separate, do you mean 3 individual windows running in one instance of the Firefox application, or 3 instances of the Firefox application?

You can set Firefox to create separate windows for ALL externally launched URLs instead of adding them to an existing window. Maybe that would work?

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste link.o and pause while the list is filtered

(3) Double-click the browser.link.open_newwindow.override.external preference and enter the desired value:

-1 = follow browser.link.open_newwindow (default) 3 = open external links in a new tab in the last active window 2 = open external links in a new window 1 = open external links in the last active tab replacing the current page

Then you only need the -url "http://dilbert.com/" part.