I think there used to be a checkbox for that in an earlier version... The following requires some tapping, and maybe there's an add-on that makes it easier, but if not: In the Awesomebar, type or paste '''about:config''' and tap the Go arrow. ''I suggest bookmarking this page for future ease of access.'' Type '''image''' into the search box and tap the Enter key to find all preferences that contain image (singular, not plural). Scroll to the bottom and modify '''permissions.default.image''' from its default value of 1 to 2. Desktop Firefox lets you make site-by-site exceptions, but I don't know whether there is a way to do that on Mobile Firefox. Note: I tried this in Firefox 20, not Firefox 21. Please report back if it no longer works.