Can be parsed through the WTS and translated online only these files:

*.properties
*.dtd
*.ent

But cannot be translated online the files below (which can be included in the submitted xpi)

*.rdf
*.js
*.html
*.xhtml
*.(other formats)

...and must be downloaded, translated offline and uploaded using the Upload link at the top or at the bottom of the locale page

Note: you have to be logged in order to access to the Web Translation System

For more information, please see the other Questions & Answers