Perhaps unsurprisingly, this bug can be fixed by making the same patch that fixes the system-level cairo library.
I manually patched firefox 8.0b3 to test. The patch goes into the cairo version that's embedded in the firefox source, at [RELEASE]/gfx/cairo/cairo/cairo/src/cairo-ft-font.c.
Perhaps unsurprisingly, this bug can be fixed by making the same patch that fixes the system-level cairo library.
I manually patched firefox 8.0b3 to test. The patch goes into the cairo version that's embedded in the firefox source, at [RELEASE] /gfx/cairo/ cairo/cairo/ src/cairo- ft-font. c.