It turns out that the bug is not related to the usage of the name "content", but rather that TYPO3 has immediately executed JavaScript that references top.content before the frameset in the file.
I assume that top.content is then initialized to some value, which the later addition of the frameset isn't allowed to override.
I have created a minimal testcase, at http:// www.martinseeba ch.dk/firefox- bug-418807- testcase/
It turns out that the bug is not related to the usage of the name "content", but rather that TYPO3 has immediately executed JavaScript that references top.content before the frameset in the file.
I assume that top.content is then initialized to some value, which the later addition of the frameset isn't allowed to override.