Translating online text doesn't get much easier than with the free gTranslate add-on for Firefox. This small, simple tool acts as a conduit to the (also free) Google Translate service.
If you select some text and then right-click it, you'll see a new Translate option. The tool will display an automatic translation of the first part of the text right next to the menu option, which will be all you need if you only choose a few words or a small sentence. For longer translations, one more click will bring up a full translation in a new browser tab. gTranslate will attempt to automatically recognize the original language used, and will by default translate into the same language your browser uses. You can change either option, but you probably won't need to.
The nice little tool can't translate a full page, so for that you'll still need to enter the page's URL on the Google Translate site. But for translating blocks of text, gTranslate is an excellent option.