I have collected some extensions which could be very helpful for students:
ScrapBook is an extension which helps you to organize interesting pages which you need for your seminars or your diploma thesis. It saves selected pages to your hard drive. Of course, all interesting attributes like original internet address and time are saved, too.
You can save the whole page (incl. images and style sheets) and only parts of a webpage. You also can mark interesting parts of the webpage and save this selection.
Customize Google is an extension, which improves the use of the famous search engine. It can add several links to each search result and add addition filter functions. The extension supports every Google tool like gmail, google calender, image search,…
Many people are using Gmail, because it is a fantastic web mail tool. Gspace enables to save files to your Gmail account. You can select every file within your firefox browser and upload it or you can download uploaded files.
Many pages use flash to show advertisements. Unfortunately, these advertisements needs a lot of memory. Firefox will slow down, if you use many tabs. Flashblock blocks every flash movie until you will activate it.