https://packages.debian.org/sid/firefox Many disagree with this stance, but Testing is in an awkward position and I generally steer people away from using it over Stable or Sid because you get downsides of both without the strengths of either. I blame Debian with shooting themselves in the foot when it comes to communicating this. Testing is not a happy middle-ground, it's a test bed for stable. Staying on Sid gives you essentially a rolling release like Arch and it's plenty "stable" for regular desktop use and should anything break you'll get the fix (usually) faster than on testing.

Sorry for going on a bit of a rant under your post, but I run into this on the regular.