A website that I was working on suddenly started crashing Firefox 3.6 on my PC (Ubuntu 9.10, 64bit). I’m talking a full out hang with 100% CPU usage and a frozen mouse and keyboard and the only way to recover is to remote login from a 2nd PC and kill the Firefox process.
I found the cause of the crash and it was a single line of recently added CSS code at the top of the websites stylesheet: body { opacity: 0.9999; }
So, if you run a website, be careful of non-100% opacity when it affects many elements. I have hosted a page that reproduces the issue and found a bug that tracks the issue: #279890. This CSS is also a problem on Windows – it causes text to blur horribly in IE 7 in some situations.
What is that CSS for anyway? Firefox 2.0 on Macs used to have a problem where text could dim or get bold or flicker when a animated effect that changed the opacity of an element was used. It was caused because the use of a opacity filter triggered the Gecko rendering engine to switch from the operating system’s method of anti-aliasing to its own internal method. Whenever opacity dropped below 100% the mode switched and resulted in a defect such as a flicker or blink. The fix was simple – all that was needed was to add body { -moz-opacity: 0.9999; } to the stylesheet because it would force Firefox to use a consistent text rendering method.
