This is a weird bug, to reproduce you must have Flash Plugin 10.1 or later installed.
How to reproduce:
- Open your GMail inbox
- Notice that all fonts are good and antialiased
- Click compose window
- Note that fonts become distorted like losing their antialiasing
- Click "Discard" to close Compose window and the fonts become normal again.
This seems to be due the Flash plugin (Compose window uses it for attachments), if you disable Flash bug will not reproduce.
Created attachment 81786 [details]
Fonts before I click on compose window
Created attachment 81787 [details]
Font distortion with compose window open
Also note that this only happens with WebKit, won't reproduce in Chrome either.
To reproduce the issue you need to enable "Rich formatting" in the compose window, if you disabled it.
*** This bug has been marked as a duplicate of bug 23364 ***