Test IE and FireFox CONSTANTLY

I can't think of anything that can be as hard and scary as working hard to get something to work only to find it fail in FireFox (or vice versa).  This has particular relevance with JavaScript, but the same is true with CSS. 

If you constantly check both browsers, you don't have to worry about later turning off ALL the code and stepping through each piece trying to figure out what to change — which can take hours rather than seconds to correct if you constantly check.

From the Mac end, it seems if stuff works in both IE and FireFox, 99.9% it will work on FireFox & Safari for Mac.  Safari has some little quirks but they are minor.

Just a little painful learning I'd thought I'd pass to you my friends.