Like most apps, uninstalling Firefox from an Apple computer is a simple matter of deleting the application file. OS X Yosemite doesn't have a complicated uninstall application. However, Firefox does have additional library files that aren't removed when you delete Firefox.app. Locating these files in Finder can be tricky, since they're stored in a hidden library folder, but Firefox's help menu can point you directly to these files.
Launch Firefox and click the "Menu" icon in the upper-right corner. Click the "Help" icon at the bottom of the drop-down menu. Select "Troubleshooting Information" from the Help menu.
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Click the "Show in Finder" button located beside Profile Folder in the Application Basics section, to open a Finder window with your Firefox profile folder selected.
Click Finder's "Column View" icon at the top of the Finder window. In this view, your highlighted Firefox profile folder is inside a Profiles folder to the left, inside a Firefox folder to the left of that. Delete the "Firefox" folder by dragging it into the Trash in the Dock.
Close all Firefox windows. Click "Applications" in the left menu of the Finder window. Delete the "Firefox" icon in the Applications folder by dragging it into Trash in the Dock.