Windows 10: Yet another windows 10 issue ASK Hijack?

Are you using the traditional Search box or the Address bar search?

There are options in the FF config file (about:config) that allow you to set a search engine for the Address bar.
Try:

• Type about:config in the Address bar
• Acknowledge the warning message
• Type search in the about:config Search box
• Look at the results and check for any that mention Ask

If you see an Ask entry, take a screenshot and post it so that someone can advise you further.

Here is a screenshot from my Pale Moon about:config page (the FF one should be fairly similar):

 

Try giving AdwCleaner a shot.