Can you find it on your downloads list? You can open that by clicking the download arrow on the main toolbar, or by pressing Ctrl+j.

If it was saved to a website instead of generating a download, you could check your history to see whether you can return to the site.

Please note: if you are filling in a PDF form on a website, and you want to be able to save the form contents that you filled in, you may need to switch from the built-in PDF viewer to the more full-featured Acrobat plugin. This article has the steps: View PDF files in Firefox or choose another viewer.

Note that if you opened the file in an external program via the "Open with" download dialog for editing that you should always use "Save as" to save the file under a different name in a different folder and not leave it in the Windows temp folder and not use a normal "Save" to replace the file to avoid losing the file and the edited content.

If you can't find the file in the download folder, look in the temp folder.