VTzilla is a Firefox extension which uses VirusTotal to scan links and files before they're downloaded.
If you're unsure about a particular link then you can right-click it, and select "Scan with VirusTotal". The URL is passed to VirusTotal, checked against 50+ top antivirus engines, and you're presented with a report on site safety in a few seconds.
That option is relatively hidden, of course, and there's a chance you'll forget to use it. Fortunately the developer understands that, and VTzilla also adds a "Scan with VirusTotal" button to the download box as a permanent reminder. One click and you'll be reading the report on any file.
Less usefully, VTzilla also adds a toolbar where you can search VirusTotal for hashes, URLs and more, or launch a scan of your current site. This grabs a lot of screen real estate and isn't customisable, but you can at least turn it off and just use the right-click option and download box button (click View > Toolbars > VirusTotal Toolbar).
The interface could be reworked to save screen real estate, but VTzilla's core functions work well, enabling you to check files with VirusTotal on demand.