To simplify life, completely uninstall any previous versions of Firefox, then install Minefield. Otherwise, you can incur profile issues unless you want to manually create multiple Mozilla profiles, which is an annoyance IMO.
If you are using Minefield, just use the auto update feature to roll to the latest nightly - works about 99% of the time, expect to occasionally force a manual install if a nightly update fails.
As for the Nightly Tester Tools, the package will usually work until a major revision status change, which is typcally a few weeks apart, after which a new Nightly Tester Tools package should be released to match the updated revision.
Alternatively, there are other ways to try installing packages and extensions that do not natively support the latest Minefield releases. For example, you can manually extract a package, edit the install script to change the revision qualifier, recompress into an install archive, then try forcing a manual installation.