If you are using the
free driver from linuxant, then your modem is restricted to 14k4 ... i.e. quite slow. Web sites that have a lot of graphical content will take a long time to load.
However... this should not chew into resources so much you cannot do anything at all. Is it the entire desktop that freezes or just firefox?
Have you tried a site with only a wee bit of graphics like LQ? Or
http://wiki.linux.net.nz/