Hi DiggerJules, It is probably best to continue this as part of the original question. I will close the second thread [/questions/1132141] and add the information to the original thread [/questions/1132062]. Please also let us have a screenshot or more information. Possibly you have malware or adware. It should be ok to let Firefox be your default browser. What are these tabs, are they something you are trying to open or download ? Have you set Firefox to open tabs from last time, if so and you had dozens of tabs open you will again open dozens of tabs until you close them. (Holding the mouse cursor over one tab on the left and repeatedly clicking should quickly remove them all one after another) ''DiggerJules [[#question-1132141|said]]'' <blockquote> After asking about this yesterday (and thank you for responding to my query), I have followed your suggestion to Change the action for a content type, I have Reset actions for all content types, and deleted mimeTypes.rdf. Now Firefox doesn't automatically create dozens of tabs - the only difference is it asks me every time because Firefox is no longer my default browser. It would still open dozens of tabs if I typed yes. It's not actually downloading anything, or if it is I don't know where it's putting it. I don't understand why this is still happening - but has only happened since upgrading to Windows 10 last weekend. Thank you. </blockquote>