See:

• https://support.mozilla.com/kb/Firefox+keeps+opening+many+tabs+or+windows

Thanks! But I guess it's a bug in the new Firefox-version. Couldn't fix it with the link.

What is the file name (file extension) of the file when this happens?

Did you delete the file mimeTypes.rdf in the Firefox Profile Folder?

• Help > Troubleshooting Information > Profile Directory: Open Containing Folder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

I do not download anything. It's a simple login procedure. The website says, "The website is opened in a new window.", then the next tab opens and it writes the same...

Clear the cache and the cookies from sites that cause problems.

"Clear the Cache":

• Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"

"Remove the Cookies" from sites causing problems:

• Tools > Options > Privacy > Cookies: "Show Cookies"

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions 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

If it does work in Safe-mode then disable all extensions and then try to find which is causing it by enabling one at a time until the problem reappears.

• Use "Disable all add-ons" on the Safe mode start window to disable all extensions.
• Close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

In Firefox 4 you can use one of these to start in Safe mode:

• Help > Restart with Add-ons Disabled
• Hold down the Shift key while double clicking the Firefox desktop shortcut (Windows)

• https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes

I'm afraid, it didn't help either. I disabled all Add-ons and Plug-ins. The cookies are deleted every time I close Firefox. I deleted the Cache as well. Didn't work.