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Fair enough, if enabling the xcb backend slows things down, especially if it is unmaintained so not likely to improve soon, we should remove it. It's a shame that awesome has to go but I'm sure the people using awesome know how to handle building from AUR/ABS and create a xcb-enabled cairo for it.

If it was just an optional backend, I would leave it enabled. But the fact is that the XCB backend replaces the Xlib version, resulting in visual bugs and crashes.

Have you tried --enable-xcb and --disable-xlib-xcb - this should keep the Xlib version as is, but still support the xcb version. I'd be willing to test this configuration if there's some reliable way to invoke the fore mentioned crashes. I don't usually use Seamonkey or Thunderbird, and my Firefox is a tarball download from Mozilla.com -- damjan