You can rebuild the icon cashe, "If you need to rebuild the Icon cache, Open File Explorer > Folder Options > Views to show Hidden System Files. Next, go to C:\Users\Username\AppData\Local folder and delete the hidden IconCache.db file. Reboot. This action will purge and rebuild the icon cache".
Click on the Start button and then in the search box type "file association". Yoou should see at the top of the list "Change the file type associated with a file extension". Open that and wait for it to be populated. Scroll down and check which files have got Firefox against them. They should be web types eg html.
Is it a particular file extension that has the Firefox icon? If so look for that file extension.