You can find the connection settings in Tools > Options > Advanced : Network : Connection
If you do not need to use a proxy to connect to internet then select "No Proxy"

See "Firefox connection settings":

• Firefox can't load websites but other browsers can

Another possible cause is that your ISP sends images compressed to speed up the transfer. Some add a JavaScript to each page to make it possible to see images in the original quality.

If you want to see the original image quality all the time then you need to contact your ISP and ask what to do to opt-out of that service or see if there is a FAQ about it on the web page of your ISP.

Thanks for the solution cor-el. I set the internet connection to "No Proxy" and images are focused again. I appreciate the information.

You're welcome

What were your connection setting?

Were you using a proxy connection with your ISP or another proxy or web acceleration service?

I have got exactly the same problem occuring here - but only since I updated to Firefox 4 this afternoon.

I have tried what you mentioned to do up there and restarted the computer but they are still horrid to look at and blurred!

Any additional help - wish I hadn't changed now!