I compared this Add-On to the recommended "To Google Translate", and this Add-On ( "Translate Web Pages" ) is far better.

This Add-On has expected behavior when user clicks on the Add-On icon. It displays a drop-down window with a "Translate" button to translate the current page, and it has an addition option button to access additional options on the fly.
User can quickly translate the page by right clicking the page and select translate to English (or your language).

The other Add-On puts a toolbar on the top of the page, where it occupies valuable webpage real-estate. Where-as this add-on keeps those options on the icon itself (IMHO that's where they belong).

It translate the page without having to re-display the page.

I highly recommend "Translate Web Pages" over the other similar add-on.