Before you get started, please remember that for the time being we should not do translation work for languages that are maintained upstream. If you want to help anyway, try to get in contact with the upstream translation team.
In any case, if there is no upstream translation its certainly a good idea to start translating in launchpad ...
Further, if there is no upstream translation team, it might be a great idea to go ahead and start one :-P. This will help you to get better recognition of your work and help ubuntu by getting more upstream visibility and it will help upstream by getting valuable contributions.
In case you gain official upstream status you can probably keep using launchpad to do the translations and regularly export and submit your work to the upstream tree.
If you need help contacting upstream, ask the MozillaTeam ... maybe we can help.
How to Start
If you want to translate a mozilla application you first have to ensure that launchpad translation support for the application/package has been set up. If this is not the case or if you are in doubt, get in contact with the MozillaTeam.
Once launchpad translation support is in place for your application, double check that your language has no official upstream translation before you finally start to translate using launchpad. Launchpad translation UI is quick to lern, efficient and supports a lot of bells-and-whistles that help translators to get their job done. Give it a try.
How to Test
Its important to test your translations before your language can be included in the ubuntu translation packs or considered for upstream submission.
If you wonder why you should test your translations, remember that in mozilla world (vs. gettext) bad translations can break the complete UI
Setting up the po2xpi tools
Export the .po files
produce and test .xpi packages from the export