This is an issue caused by the ClearURLs add-on that will need their authors to fix it.
As a work around, you can disable the add-on, or perform your search directly from the address bar in Firefox.
The way Firefox detects these websites is through information that is posted on the website itself (called "OpenSearch"). If the website posts it, and it is valid, then Firefox can use it to provide a search engine offering to the user.
On https://ww...
The "browser.bookmarks.editDialog.showForNewBookmarks" preference is equivalent to the "Show editor when saving" checkbox that is on the pop-up dialog.
This is typically due to third-party software that has somehow been installed on you machine, typically called adware. Try using a product like Malwarebytes to remove it.
