The Problem and Solution

If you try to upload an image or video to a site via Firefox and see a weird striped-rainbow pattern instead of what you intended to upload then the fix is actually extremely easy:

1. Type about:config into the address bar and hit enter
2. Ignore spooky message and click big blue button
3. Search for privacy.resistFingerPrinting and set it to false.

What was happening? When this setting is enabled Firefox randomizes HTML canvas data. Instead of an image you get the ugliest plaid you’ve ever seen.