In case the link breaks, here are the instructions:
To disable images in Firefox V 28.0
Type "about:config" in the URL bar (without quotation marks), and click on "I'll be careful, I promise!". This lets you get to the about:config menu.
in the searchbox, type in "permissions.default.image". you should see this:
Now, you'll want to right-click it, and select Modify. The values 1,2 and 3 are used in this argument box; 1 being allow images, 2 disabling images completely and 3 if you want to allow images loading from the same website but want to disable 3rd party images. For this instance, we want to make it's value 2, to disable images completely.
Press Enter, or click Ok to set the value.
You may also want to right-click on "services.sync.perfers.permissions.default.image", and select Toggle to turn it's value to False, as it may try and reset these parameters when you re-open Firefox.
This process is about the same as the first, except we're searching for something else.
Type "about:config" in the URL bar (without quotes), and click "I'll be careful, I promise!" again if needed.
And there you are! I hope that helped you out!