Disabling images from loading in  your Firefox browser is pretty a simple way to make your web surfing faster and hassle free and also way of conserving the limited bandwidth allotted you by your Internet service provider.Disabling images from loading does not require any firefox add on .Just follow these few sreps to get your Firefox images disabled
Launch your Firefox and go to TOOL/OPTION/SETTING  from the menu bar.

Click on the CONTENT TAB  and deselect  the check box   next to LOAD IMAGE AUTOMATICALLY and  click ok

Alternatively you can choose to block or disable images from loaded on some site by selecting the EXCEPTION  tab as shown above  where you can add the URL of the site you want to block
That is it .The next time you load your firefox images would be disabled

 Or a better solution would be to use the Image Block extension which will allow you to selectively block images on certain sites.

To enable the image to load,follow the same process and tick the check box next to load images automatically

I hope this helps.
If you know any other Firefox trick you can recommend, feel free to share.