Interesting question--I've used Firefox for awhile now, and haven't had any issues with PTO Manager. But I haven't had any need to edit the Content Manager stuff for awhile, either. I just went in to edit something and now know what you are talking about. I keep IE on my computer for the few rare instances I run into when something is incompatible with Firefox, especially with the v3 upgrade. So, if I need to change anything in Content Manager, I'd just temporarily switch over.
But I agree with you--the whole thing s/b at least Firefox compatible. I run into more and more people leaving IE, mostly for Firefox. Will be interesting to see how many start using Chrome, too--as much of a Google fan as I am, I haven't played with Chrome at all yet.
We transitioned away from using Internet Explorer to Mozilla products some time ago due to the security issues with IE. This is the first time I've run into a web based product that isn't browser neutral on the admin side and wondered in the case of Content Manager Overrides or Linux or Mac users what you have done using the product? Quite a few are Mac users but even those on Windows versions no longer use IE.