Firefox for iOS has a mode where it hides/blocks images if you have a slow connection or want to avoid using a lot of data.
How to Block or Show Images
Tap the menu button at the bottom of the screen (the menu will be at the top right if you're using an iPad):
- To hide images, toggle the Block Images switch (Hide Images for older versions) to block images and not load images while browsing. A blue dot will appear when this is turned on, toggle again to turn this off.