NoScript & FlashGot both have & have had a META-INF section with associated manifest.mf, zigbert.rsa, & zigbert.sf files within.
Or is it more to it then that?
(Modify a file within the xpi, leaving the above mentioned files unchanged, & you will get complaints from your browser when you go to install.)
Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.1.19) Gecko/20110420 SeaMonkey/2.0.14 Pinball NoScript FlashGot AdblockPlus
Mozilla/5.0 (Windows NT 6.1; rv:7.0) Gecko/20110905 Firefox/7.0 SeaMonkey/2.4