I just finished the first stage of a new site. I pretty much used firefox all the way to check and test it. Now I'm getting reports from users that it loads very slow in IE. Are there any rules of thumb to follow to optimize for both firefox and IE?
I always build for IE and work out the bugs in Firefox. Since most users use IE I think it's smarter to do it that way.
I wouldn't recommend building to suit for IE first, because you're starting on the wrong foot and putting buggy code into your work. How would it look in Opera, Safari and what about the soon to be release IE7? Slow loading sounds like you're doing something else like using JavaScript or something similar. CSS/XHTML based sites alone won't particularly load any faster or slower on other browsers, factoring the browsers usual response time.
What i do is i do something to the page and view it in both IE and in Firefox very often that way i know what's causing the problem when something does not look right in either one of them.
I do it the other way... lol. oh well, Firefox will probably expand in the future to become universal.
I just try to start out with good clean code, then I go back and break stuff to make it look right in both browsers. I check with Firefox as I go, but each time I make a major change, I check with both Firefox and IE (or I just use the IE Tab Firefox extension and toggle between two tabs).