Translation of Mitschüler

Can you translate: Are you using the firefox browser? Install this tool in your browser window. The ones I suggested the other day are the most useful ones, because we would use it for generic terms which differ from Spanish.
Could you please read the following offers in a critical manner and confirm it before I will send it? I have also mentioned the warranty and the software in the offer, because it was requested by the customer.
Today I have a lot of time because I've just returned from a 3 day vacation. For how long have you been teaching German? For about 3 years I have been studying Spanish mostly alone, with books and stuff.