You can do this two ways:
Check out files from Mozilla CVS. If you don’t want to checkout all files do:
make -f client.mk l10n-checkout
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 get the correct files for en-US. To do this we will create en-US as a pseudo language.
make -f tools/l10n/l10n.mk create-en-US
This will move the correct en-US files to l10n/en-US. You can now create POT files as follows:
moz2po -P l10n/en-US l10n/pot
This will create the POT files in l10n/pot using the American English files from en-US. 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
You now have a set of POT files in pot.