We are testing the maximum of platforms, but probably not all. Always
the same problem between volume and very low numbers.
Absolutely understood and agreed. Just making the point that you can't expect *everything* to work.
Of course,
testing firefox under Linux is important, but this is representing a
very low minority of our users.
I don't know how "important" I'd call that, myself, except maybe for those products that you sell that support Linux directly. Now, which ones are those again? 
Our old site was lighter and probably faster (?), but it was also a big
source of complains from our end-users. The latest surveys we run are
showing positive feedbacks on the new design, especially finding
information or for our support site, so not everything is bad.
Do you know the saying "Familiarity Breeds Contempt"? In 3-5 years, you'll be saying how this site is a big source of complaints, and that the NEW NEW design will be receiving positive feedback...
it's also complex to use some recent web technologies
Especially when the browser that you're targeting DOESN'T PROPERLY SUPPORT those recent web technologies...
Our webmanager is following this discussion, so hope it will help
Have 'em open the website in Firefox, do a ctrl-shift-J (or Tools/Error Console) and check out the warnings. Some things probably aren't displaying as they were intended, and that may be a bit hidden because IE seems to be a bit more lax about accepting malformed property values as compared to Firefox, Konqueror, Safari, or Opera. (Do the same thing on this message board -- some neat warnings/errors there. What is a "VISTED" class, for example?).
Anyhow, I'm not saying that this isn't good enough. The old site *was* lighter and faster (in my experience, anyway), but it wasn't a "showcase". I can see why the product site would have a market-driven need to be kept "fresh and exciting", and this rewrite is nothing if not that.
(The support site, however? I don't see the big benefit, myself, but that's just me. For example: I don't need to be entertained or marketed to when looking through the FAQ list, I just need the FAQ search to show up and be usable--anything else is a distraction. And while it may be "pretty", pages such as your FAQ system that cram all of the content into a fixed-width column down the center of the screen are annoying. I run a 1920x1200 screen, and the actual FAQ's text column is just barely 25% of the width of my display. It is USABLE -- I'm not arguing that point -- but the website design certainly places artificial constraints on the presentation of information that don't really need to be enforced.)
So -- I don't think that you have too much of an issue to deal with, myself. Just keep picking at stuff, and it'll eventually get straightened out. In the meantime, if you aren't trying to sell to a certain class of user, it's not anyone's business but your own if you don't make your initial build of the website work properly with that user's browser. If the cost of supporting Firefox on Linux exceeds the sales that you'd get from FF+Linux, or from IPod Touch users, etc. it becomes a pretty simple equation--go live when you WANT to and fix the site later, if you get around to it. That seems to be the way things work, anyhow.
BittMann