Hi Qev,
I have the same problem, and my computer is wired to the router. I have a DIR 625.
I can access it via Netscape, however with Firefox does exactly what it does to you.
It also reboots when downloading bittorrents. This is apparently related to a buffer overflow issue they were having with the dlink routers since long time ago, I guess.
Try accessing the page with another browser.
The funny thing is that on another computer, also wired, and on all the wireless computers in my home I can access the configuration page of the router w/o any problems, and I have Firefox 3 on all of them, so I do not think necessarily that it is a Firefox issue, though it could be.
I would like to hear some competent technical answer to that if anyone could give it to me.
Cheers.