See if this helps you. https://support.mozilla.org/en-US/kb/firefox-repeatedly-opens-empty-tabs-or-windows

Thank you Edmeister, it has done the trick indeed. Do I need to do something with the old profile folder?

You get tabs opening endlessly if you select the Firefox program to handle a file when you get an "Open with" dialog. You should never select Firefox as the application to handle a file when you get the "Open with" download dialog.

You can rename/remove the mimeTypes.rdf file in the Firefox profile folder to reset all download actions. You can use this button to go to the current Firefox profile folder:

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

Found it. Thank you for the advice. I'll bear it in mind. The new profile folder also has the mime etc folder. It has been newly made/reset by following the edmeister's suggestion's sugestions (resetting Firefox). I figure I don't need anything from the old profile folder any more, and I can remove it.

If you don't need any of the user data files in that old Profile . then delete it via the Profile Manager. That will remove both "Profile" folders (2nd one has the Cache and other transitory files) and remove that Profile from the profiles.ini file.

I don't know if I need anything in that folder. The changes I made before can be made again to Firefox, according to the current needs. So, the simpler thing to do is remove it. But what is the Profile Manager and where do I find it and how do I make it work for me?

I found the Profile Manager, but the profile folder it presents to be removed is named "default14.....", while the old profile folder on the desktop, waiting to be removed, is called "b16umkls.default". Shall I therefore remove this folder by hand and forget about that other folder with Cache and transitory files you mentioned and also about the .ini files?