I have found that between firebug and chrome you can generally nail down most bugs. What one does not catch the other does. IE is a whole different beast you have to have visual studio to do any decent level of debugging and it is still painfull. That said sticking to a toolkit eliminates a good deal of the cross platform cross browser issues. 9 times out of 10 when we do have an issue it is css rather than javascript hat bites us. We use Dojo or jQuery for projects depending on size and both are good at smoothing out the inconsistancies as long as you color in the lines.