Okay, this is a weird one. We have a custom page where we've reorganized the fields on the Contact object. We've essentially implemented our own quick edit solution with Javascript. When a user switches the page to edit mode (which just rerenders the container with the fields, swapping outputFields for inputFields) Firefox lags horribly when clicking from one field to another. There's probably a full two to three second delay between a click and the corresponding focus change.

I don't even really know how to go about troubleshooting this. In Chrome and Safari it's instantaneous, as you would expect. Some of the fields I'm showing on the page use fieldsets and some don't. I've tried commenting those out to see if it was stuck in some sort of loop. I tried showing the edit version by default (rather than the read version) thinking it might be something with that switch in JS. So far nothing has changed the behavior. 

Is this just a known problem that we haven't stumbled onto until now? Or is something seriously wrong that I should be able to find and fix?