Why do you suppose they used cartoonish icons for the browsers like firefox and opera? Is it just to make it look more friendly to a naive user? I assume it

Why do you suppose they used cartoonish icons for the browsers like firefox and opera? Is it just to make it look more friendly to a naive user? I assume it has nothing to do with any sort of trademark issues but I've wondered what the limitations are for using those icons on a website (for instance, a "Best Viewed By These Browsers" box).