Programs for which currency is particularly desireable, such as Firefox, should be downloaded using apt-get as soon as they are available in Sid. Once that's done, change your sources.list back to testing, or whatever, and you won't have to worry about it any more.
Doesn't specifically address your problem, but it's a method I find satisfactory. If you want to mess with it, you could #dpkg remove --purge firefox ... then apt-get 1.5 back from Sid, and your problem is solved.
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