User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; fr; rv:1.9.2.15) Gecko/20110303 Firefox/3.6.15 Build Identifier: Mozilla/5.0 (X11; Linux x86_64; rv:2.0.1) Gecko/20100101 Firefox/4.0.1 It looks like most .png files used within Firefox itself have just been saved using Adobe ImageReady, these files could actually be 10% smaller (or even more). There are a few well-known tools like OptiPNG, CryoPNG, PNGOUTWin, DeflOpt that are used by skilled web designers to reduce the weight of PNG files to speed-up loading time. I have applied these tools to the files in the branding/firefox/content/ directory, here are the results (size is in bytes, -o is the optimized version): 55620 about-logo.png 49858 about-logo-o.png 48917 about.png 45253 about-o.png 9395 icon64.png 8385 icon64-o.png 5640 icon48.png 5096 icon48-o.png 2659 about-wordmark.png 1559 about-wordmark-o.png Reproducible: Always Steps to Reproduce: 1. Apply any optimization tool. Actual Results: PNG files end up smaller. Expected Results: Optimized PNG files should be included.