Nothing you really need to (pre)worry about as FF has some of the best js support of the current range of browsers.
Just try to steer clear of using proprietory methods such as document.all and you should be ok.
Stick to using DOM 0 and W3C DOM methods and you shouldn't hit too many snags.
You should still test in FF (and others) to make sure your script works for sure.
Rather than list any and every potential problem here, it's far easier for us if you create your script, find out if it works in your targeted browsers. Then, if it doesn't work, post a query asking for help getting the problem sorted out - preferably with either the code in question pasted into the post or (even better) by posting a link to the site with the current, non-working js where we can see exactly what's going on in both your js and any other code which may influence whether your js works or not.
Last edited by Bill Posters; 05-04-2006 at 08:38 AM..