Essentially the same problem as bug 13800: search fields may share the cancel button style, but they should not, because they mutate it independently of each other. I have a patch that includes a test case.
Created attachment 14718 [details] Avoid sharing the cancel button style Search fields use a few other pseudo element styles (for the results widget), but since they don't mutate them after creation, I don't see any risk in that.
Comment on attachment 14718 [details] Avoid sharing the cancel button style r=me
Landed in r21767.