That doesn't matter which browser you are using, the one thing you need to know is how to open "Inspect Element" on it. You can even search on Google how to do that if you don't know. In this tutorial, i'm using Chrome browser.
1) First of all log into your Instagram account and after navigating to that video page you want to download.
2) Open that video, now right click on it and in Chrome "Inspect Element" is in the last line, select it.
3) After that, you need to search for a link to mp4 file in inspect element opened area. After you found a link, double click on it and copy it.
4) Now open a new tab in your browser, paste that link and open.
In that page, you will see only that video. Right-click on it and select "Save video as..." line.
5) And at last in opened window select the direction, where you want to save your video.