Firefox from the Linux Mint repo is at version 17 (and so is thunderbird). Iceweasel in Testing and Unstable are the Extended Support Releases (ESR), and they are up to date AFAIK (I think both are 10.0.11 right now). If you want to go back to the standard releases, you have a few options. First, you can install from the Mint Repo, which would get you firefox 17. You could also use apt-pinning and pull iceweasel down from experimental. Finally, you could try to use other distros repos to get thunderbird, but I wouldn't recommend this. Currently, your preferences will work, but it could cause problems in the future if packages from the mint repo stop working with testing. It would be better to assign them all equal priority or to delete the contents. Hunspell is incompatible with Thunderbird (but not icedove), so if you were to install from the mint repo, you'd have to remove hunspell which is important for spell checking.
If you do plan on installing iceweasel from experimental, make sure you remove the third entry (the debian one) because that will assign all debian releases a pin of 500, meaning you would upgrade everything to experimental, which would be bad unless you wanted to.
Edit: the link that zerozero gave recommends using experimental for the latest release of iceweasel, and it gives instructions.
Dell XPS 15 l502x - Debian Testing 64-bit NetInst Xfce, SolydX 64-bit Debian Testing, SolydK 64-bit SolydXK Testing
Old Gateway Pentium 4 Desktop - Arch Linux 64-bit Xfce and SolydX 32-bit Sid