What Options Available For Deep Source Locating Where/How Someone is "Sync'd-Link'd/Attch'd within My Accounts, Mozilla, Firefox.. (Not Sure)

Some how someone has attached to my computer and its system or programs, accounts Etc... Im not entirely sure, I am Looking for anything that I can Install or Add-On, Download or Extension's, Whatever Could Help Deep Search and Locate the Root Cause or Area where they were able to attach to my Files or whatever it is they latched onto.