I have no experience of usb mobile modems, but my understanding is that there are IPv6 protocols, period. Devices either support them, not at all or wrongly.
I was caught with the IPv6 issue some 4+ years ago when I first started using Linux. At that time IPv6 was enabled in most Linux distros but not in Windows XP. Long story short" the problem was solved when I updated my router firmware. The old firmware simply didn't understand IPv6 and was confused by the traffic emanating from Linux - even though my Linux box was trying to access IPv4-address webpages. Pass.
That being said, I doubt your problem is really an IPv6 issue per se because IPv6 is enabled by default in Vista, Win 7 and the last three iterations (at least) of MacOSX. That modem just has to work with Vista, W7 and MacOS. So - I wonder if this is an obscure bug with IPv6 implementation in Ubuntu which is only revealed with that particular modem, or is an entirely unrelated bug that is coincidentally bypassed when you disable IPv6 in Firefox.
Sorry - I'm not being much help here. I'm out of my depth and I get the impresssion I'm beginning to burble without making any sense.
One experiment you might try if you wish. Before I upgraded my old router firmware I found that setting a static IP address in my Linux box and configuring DNS addresses manually got round the problem. I've read elsewhere that the IPv6 issue can interfere with name resolution, so that makes some sort of sense. You could try that but it may not get you much further.
By the way - out of interest - do Update Manager and Synaptic show any delay in resolving IP addresses when you use them>