OK, let's compare google translate to that moztrans thing you mentioned. I have just downloaded it and tried it with a few paragraphs but more abaout the quality I will speak another time. Let's just outline its advantages to google. As you know google translate is web based service and you need to open it in another tab. Moztrans is an add-on for Mozilla Firefox (and seamonkey) and you don't need to open anything else. The advantage of this is that you only mark the paragraph and choose the option "Moztrans" after you click on it with the right button of your mouse. The translated text shows in a small window on this tab and you do not need to shift between tabs.
The disadvantage of moztrans is that it loads quite slowly when you change the target and sourse language (might be because it is still experimental0 but I found the clue for this. Close the moztrans window and then do the same thing again -the result shows up in a second.
But anyway, do you think that this could be useful? If one has to reply to business emails they usually use emails based in the company's website. If someone talks business to me from a google or yahoo email I would just consider that person spam.