In Firefox, you can disable right click hijacking by doing the following:
- Enter "about:config" (without quote marks) in the URL bar to get to the advanced settings.
- Search for "dom.event.contextmenu.enabled" (again, without quote marks)
- Set dom.event.contextmenu.enabled to FALSE
- Not sure if you have to restart Firefox but you probably have to at least reload the page.
What this does is it enables the right click menu for images and videos, allowing you to save them as you normally would (with "Save image/video as..."). Of course, this method be slightly tedious if you have to save a lot of images or videos.
This may break sites that relies on right click input but you can always change back the setting if you need to.
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There is software for mass downloading or "scraping" but I have no experience with it and can't really help (but a Google search will help). Use software at your own risk, from what I read there are some Chrome extensions that you want to avoid.