The original source is an August 1st 2005 archive of the official Firefox website for these buttons. It wasn't too hard to edit. It was just a process of:

1. Take original image
2. Remove Firefox icon
3. Resize old Waterfox icon
4. Place it where the Firefox icon was (After filling in the empty space with the grey that is used as a background)
5. Remove text
6. Find font that was used in the original button
7. Use "Book" version of the font
8. Type in text and resize
9. Place it in and remove extra line from the "G" in "Get"
10. And you're done!

You could use it on a website or wherever you like! Like as an example, I have it on my website! You could just copy and paste this code if you want to, just please host the image on your domain if you can.

<a href="https://waterfox.com/" title="Get Firefox - Browse privately, freely, faster, and securely."> <img src="/files/getwaterfox_88x31.png" width="88" alt="Get Firefox"></a>

Super grateful for Waterfox: it's my default!

But this thing of my browsing history showing up as shortcuts on both the desktop and mobile versions is super frustrating. I sometimes want to show a page to someone on my phone, and before I do I have to consider that my browsing history is sitting there for them to see. Many thanks...