If it's a typical application bundle (like what you see in the Applications folder), then just drag it to the trash.
If it's a disk image that you're looking at (some applications come bundled in that), click the eject icon in Finder, and drag the disk image to the trash.
You can then empty the trash at your convenience; either open it by clicking on the trashcan icon, or you can press and hold the control key while clicking, and select Empty Trash.
If it's something else, provide a pointer to the download and somebody will have a look.
Thank you for answering. After transferring "My Documents" folder from my PC i was not able to open them. I was told to use "Open Office" and to download it using "Firefox". I am now able to open all document from My Document folder and no longer have any use for "Firefox"which i want to un-install. .
Drag the Firefox icon to the trash.
That was a somewhat odd sequence, as well. You could have downloaded LibreOffice or NeoOffice packages or one of the other packages using the Safari web browser; using the built-in web browser. There should have been no need to download Firefox here. I'd accordingly approach your existing source of information on operating OS X with some skepticism, in other words.
The Apple iWork Pages, Keynote and Numbers tools can open office files, as well. They're also faster than LibreOffice and NeoOffice on OS X, in my experience, and better integrated with OS X and iCloud.