wet front driver side floorboards on my 2010 Mercury Mariner

Do you have a sunroof? If so there may be a drain tube that isn't connected or its torn or broken. You will need a hands on inspection to find the root cause. The shop will likely wet the car with a hose to find the leak. Here is a link to find a shop near you. https://repairpal.com/repair-shops Alex

Likely windshield seal leaking - it's not a DIY repair in any event.