Abrowser don't find loggin page

Login screen? You mean a captive portal? In Abrowser, captive portal detection is disabled by default, which might cause your problem.
There used to be a checkbox on the main page that allowed you to enable portal detection, but it seems to be gone now.

You could try enabling it manually: Go to Abrowser's settings page by entering "about:config" in the address bar, then click past the warning.
Now you just need to set the property "captivedetect.canonicalURL" to the value "http://detectportal.firefox.com/success.txt" (without the quotes, of course) and the property "network.captive-portal-service.enabled" to the value "true". This will try to make a connection to detectportal.firefox.com each time you use Abrowser to check if there are any captive portals.

Does it find the captive portal if you do that?

I did as you said. But now I can not even start Abrowser. The icon comes to the panel
with the text "Starting web browser" and then disappear. I tried with restarting the computer but it didn't work. How can I set back things to what it was?

Ah, I remember that there was a bug where, if you enable portal detection, Abrowser would always crash on startup, but only for some people. Apparently it is still there. In that case, your only option would be to keep a second Browser on your system for captive portals until the bug is fixed.