Testcase (note that it CRASHES unless you use build with bug 638937 fixed): layout/generic/crashtests/first-letter-638937.html When resizing the window, sometimes the first-letter ends up in a different column than the line of text it's supposed to be the first-letter for. The placeholder and the float is in different blocks. See bug 638937 for a dump of the frame tree. When the bug occurs, the last line in the column before the one with the first-letter is rendered right across the entire column set.

This, like bug 415506, should be addressed by making floating :first-letter be implemented as its own concept, separate from float.