This userscript adds a simple download-link to Instagram Videos video pages.
Even 15 seconds of gif-like video might be worth keeping...
As a plus, it also adds a "Save"-Button to Instagrams/Photos (also for multi-image posts!).
-- brought to you by the nice people at Lumerias, your video search engine.
(Please note: This add-on *only works* if you open one video/instagram via its direct URL each in a new tab/window; and not when you use the left-right navigation. For example, from a user's image stream page, use right-click > "open in new tab".)