I know how to reset this way(bookmarks):

Go to about:config.

• About:config

Search for this option "Browser.bookmarks.restore", then change its value to True, restart Firefox, now your bookmarks already change to default, but you go again to the about:config and change that option to false, because when you close again the Firefox will restore the bookmarks again

More info here:

• Mozilla Zine

Or you can reset settings(bookmark and setting):

• Reseting Preferences

I'm not sure what you mean with that screenshot that shows an open with dialog.

You did post the correct screenshot?

You can check for problems caused by a corrupted localstore.rdf file.

• http://kb.mozillazine.org/Corrupt_localstore.rdf

Yes, how do I restore this dialog to it's default position?

Same problem here http://tinyurl.com/o5j3eh2 was fixed through "Safe mode" and then "Reset toolbars and controls", but in 26.0 there's no such option.

Did you try to delete the localstore.rdf file in the Firefox profile folder?

That should do the same as "Reset toolbars and controls" did.

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

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)

If it is about resetting download actions then you can 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