Google Translate has the following URL format:
Where "de" is the language you're translating from, "en" is the language you're translating to, and "%s" represents the string to translate.
This makes it very easy to create Firefox Quicksearches for various translations. You can find the short code for the language you want by changing the selected languages below the text area. When you select a different language and click off the drop down box the URL will update.
Quicksearches are probably a lesser known feature of Firefox, despite the fact that they've been around for many years. If you right click in a search box on a webpage, the context menu will give you the option to "add a keyword for this search". You then enter a keyword and Firefox will save this special kind of bookmark. To use it, enter the keyword followed by your search term in the address bar.
The Google Translate text input box doesn't produce the same context menu but you can still create a Quicksearch. Simply create a new bookmark through the organiser or the bookmarks context menu, enter a name, a keyword and the location will be a similar URL to the one mentioned at the start of this post.
So if you want to translate some text, enter your keyword in the address bar and copy/paste the text. Quick and easy.