What is on these unwanted tabs/windows? Ads?

You may have ad / mal-ware. Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

Did you install the BankID Security Application? https://install.bankid.com/ The browser you posted here with doesn't show a Firefox add-on. Is one needed?

What happens when you test BankID? https://test.bankid.com/

Opening endless tabs can be caused by directing Firefox to open a type of file that it can't actually handle. This article has more information on that problem: Firefox repeatedly opens empty tabs or windows after you click on a link.

I couldn't read the BankID website. Does it use a plugin, or how exactly does it integrate with Firefox? If it does add a plugin, make sure it is set to "Always Activate" so that it can be used seamlessly. You can set plugin permissions on the Add-ons page. Either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Plugins. On the right side, you can find plugins that embed into sites, which you often want to set to "Ask to Activate", plugins that work in the background, which often work best set to "Always Activate", and mystery plugins that have no apparent value, which you can set to "Never Activate".

Sometimes plugin permission changes do not take effect in the current window(s), but only in new windows.

Try to rename (or delete) the mimeTypes.rdf file in the Firefox profile folder to reset all file actions.

• http://kb.mozillazine.org/mimeTypes.rdf
• http://kb.mozillazine.org/File_types_and_download_actions#Resetting_download_actions

You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox