There are a few different ones that are okay. If you like chrome you can also try the torch browser. It runs off chromium technology so it's pretty similar, and it has downloading capabilities built in.
Almost all extensions don't. Try keepvid.com its great, all you have to do it copy the video link into the bar, click download, choose the mp4 version ( any quality ) and choose where you want the video to be downloaded onto your computer. I've used it for a long time and there's no viruses.
Use Zippy Video Downloader: http://zippyvideodownloader.cu.cc/ Zippy Video Downloader is a unique, simple and fast tool. Where, no Java installation will be needed to find the video download links. You can select your favorite video format (FLV, MP4, 3GP and WebM) and quality from a simple radio list with a direct link to start download. (refer to http://top-pop-websites.blogspot.com/)