That's one cautionary note making the rounds along with a popular new extension for Firefox that lets people customize Web pages they visit without the knowledge or cooperation of Web publishers. The extension, dubbed Greasemonkey, lets people run what's known as a "user script," which alters a Web page as the page is downloaded. That capability has gained the extension an avid following of Web surfers who want to customize the sites they visit, removing design glitches and stripping sites of ads. But the extension comes with substantial security risks and could stir trouble among site owners who object to individual, custom redesigns of their pages. "Publishers for now seem to accept that it's OK for users to make some changes," said Danny Sullivan, editor of Search Engine Watch. "I can tell my browser not to run JavaScript, for example, and that could override what the publisher wants the page to do. But people are still struggling with where the line is. Some of these things may go to court, but I think in the long run publishers...will adapt...or develop other ways to combat it."