Any update from a Firefox version to the current version that wasn't the previous Firefox version requires a full update. The latest Firefox 3.6.x version is 3.6.23, so it doesn't make that much difference to update to Firefox 7.0.1 (8 MB versus 13 MB) and that only needs to be done once.

It is vital not to miss any minor updates if you are on a dial-up connection because those are usually small (about 1 or 2 MB at most on Windows). If you miss or skip a minor update then you always need to download the full version.