You can do this two ways:
Check out files from Mozilla CVS. There are some complicated instructions if you don’t want to get all the files.
The English files are in the mozilla/ module, while the translated files all reside in the l10n/ module. They have different structure but not enough to kill you.
Once you have checked out mozilla/ you will need to run the mozcvs2po scipt.
The program reads itself. You’ll notice at the top that it has instructions on where to move files. Run it as follows:
./mozcvs2pot F mozilla l10n/pot
This will extract files for Firefox form mozilla/ and place them in l10n/pot.
You now have a set of POT files that you can use for translation or updating your existing PO files.
Download an XPI file and run the following:
moz2po -P en-US.xpi pot