Currently, Debian Testing (from which LMDE is based off of) is in a state of deep freeze due to the upcoming release of Debian Squeeze as stable. So for the last few months, very few packages have been updated, except to fix bugs and the like. Once the freeze is over, updates will start flowing in again and LMDE will continue to roll along.
As for Firefox, it'll be updated when the Mint devs have time to release an update for it, I suppose. (Firefox isn't actually in the Debian repos.)
Debian Testing x64/LM9 Main x64/Windows 7 x64 - LG R580 laptop w/ Intel Core 2 Duo T6500 2.1 GHz, 4 GB DDR2 RAM, Nvidia Geforce G 105M, Ralink rt2860 802.11n, 300 GB WD HD 5400 rpm