Will updated versions of Firefox land in old Ubuntu releases, such as Lucid and Maverick, when they are released by Mozilla?
Yes, while a release of Ubuntu continues to be supported, it will receive the updated versions of Firefox.
Note that most Ubuntu releases are supported for 18 months after release. For example 10.10 is over 18 months old, and will not receive security updates, such as new versions of Firefox. There are some Ubuntu releases that are LTS (Long Term Support) releases which will continue to be supported for three or five years.
The graph below (from Wikipedia) shows the support for various releases. Yellow is the standard 18 months. Orange is the extended support on the Desktop. (Red is extended support on the Server only, this does not include Firefox.)
New Firefox versions will become your default browser if Firefox is your default browser, and you install the updates available for your system.
The browser that comes with 11.04 is FireFox 4.0 and while it is the default it is not the latest version. You can update it if you like, and as long as you didn't change the default from firefox, you will have the latest FireFox as your default after the update. But this is not exactly automatic by any stretch.