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Migrated to https://ubuntuincident.wordpress.com/2010/11/04/firefox-scales-images-poorly/.
Problem
I've had this problem for a very long time. When I zoom in on a page with "CTRL - +", images get pixelated. Example:
Solution
It seems that it's a known bug… Install the package firefox-smooth-scaling.
For a step by step guide on how to install it, refer to this page.