Can you post a URL and how you type it in exactly? After entering the url do you hit ENTER on your keyboard? A bit more specifics needed, thanks.

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

A possible cause is a problem with the file places.sqlite that stores the bookmarks and the history.

• http://kb.mozillazine.org/Bookmarks_history_and_toolbar_buttons_not_working_-_Firefox
• https://support.mozilla.com/kb/Bookmarks+not+saved#w_places-database-file

The problem is solved. Cause was that my computer has three network cards and when I switch to another card address bar of FireFox and ftp does not work. What is the cause of this behavior - I have no idea

Are you switching the network card with Firefox open or is Firefox closed when you do that?

If Firefox is open does it work if you close and restart Firefox or make network changes in Options?