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. Anyway, when dealing with html or xml files, it is much safer to attach them separately to a post in the extension topic when translated.

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

For more information, please see the other Questions & Answers