If you are a GNOME-Do user, you may have noticed that the Mozilla Thunderbird® icon doesn't look nice when appearing in GNOME-Do. That's because the logo is upscaled to fit the size of the Do bar. That scaling is made necessary, because the icon-size of the icon that Thunderbird puts into
/usr/share/pixmaps/ by default is too small. To get a crisp logo in Do the logo should be about 128x128px (like the firefox.png you might find in pixmaps). To replace the thunderbird.png with a larger icon you can take the 256x256px icon from
Copy that file into your home-directory (or somwhere where you have write permissions). Then use GIMP to scale it down to 128px. After saving and renaming the file to thunderbird.png you can then copy it as root to /user/share/pixmaps/ replacing the old icon.
sudo cp thunderbird.png /usr/share/pixmaps/
After restarting GNOME-Do, you should see a nice crisp Thunderbird icon. Like that: