LMDE consists of two groups of packages:
Packages that come directly from Debian testing repositories: These packages are only updated when Update packages are released (if a really important security risk is detected, the LMDE team might release the patch to fix it as an exception; but generally speaking LMDE users only get security or any other kind of updates for these packages when the UP are released ). This is the biggest and most important group.
Packages that come from Linux MInt Debian repository: These packages (that include Mozilla Firefox) get all kind of updates more often. You can find a list of these packages here