This isn't completely accurate. Believe me, they can (and will if they have a reason to) look for the fingerprint for type of url requested and passed across their network (basically think of it like the difference between m.phandroid.com vs
www.phandroid.com- although nowadays it is much more advanced than that). The coding in most/many html websites tailor the displayed page to the device and application requesting it, and records the version sent across through the redirects and access requests that pass across the network. AT&T busted my friend four years ago when he was tethering with his Nokia phone as his primary internet connection (he thought it was failsafe, although he used anywhere between 6 and 9 gigabytes a month back then, so he made himself a target for investigation), and this is exactly how they busted him. They sent him a letter with his bill detailing the page requests and showing all the non-mobile finger-printed urls and access requests, and they wanted something like $4,000 for three months' worth of back charges. He fought it and got it reduced down to like $2000 or so, but the point is they can tell both the device and application, and I am cautioning that this will probably be how they recognize the webtop firefox use.
That being said, I tether as well, I just make sure it is kept below 4-ish gigs or so a month (try to fly just under the radar)