Operating a single DSL line; direct to a desktop PC operating Win Xp. with a parallel feed via wireless router to a Win 7 laptop.
Both PC's operate normally on everything; SW, browsers, web pages, if Firefox is not used with Win 7 on the Laptop.
When Firefox 18.0 is used with Win. 7 on laptop - both PC's show slowness, freezing etc.
Try to boot the computer in Windows Safe mode with network support (press F8 on the boot screen) as a test to see if that helps.
Create a new profile as a test to check if your current profile is causing the problems.
See "Creating a profile":
If the new profile works then you can transfer some files from an existing profile to the new profile, but be careful not to copy corrupted files.