Google's Youtube policy is that we are not allowed to save their videos on our machines. If you need to view later (or slower speeds, rewinds, etc), use the add-ons that are allowed with Chromium, Opera or Firefox-types of browsers.
All web browsers are clumsy and inefficient, unless you use the correct choice of add-ons. My preference is Opera, since it does not have thousands of choices of add-ons, nor lots of useless and replicated add-ons. Inbuilt to Opera, with need add-ons are: Tab-Close-Undo, Double-click Tab-close, Popup-blocker, Script-blocker, Page-view zoom, Tab-previews, Background tab-openings, instant MHT-file saves, page-source views, address-copy, form-filling,etc.
Opera versions later than 12.xx are similar to Chromium, etc -- bad and narrow selection of add-ons. That's why Opera still offers versions 12.xx. Firefox is still trying to emulate the performance of Opera 12.xx, but its add-ons are messy: poor functioning, no compatible with latest versions of Firefox, & so many similar, overlapping choices. See my site for more detailed comments. Soon - I hope. gregzeng.com