I'm not usually one to weigh in on the browser holy war, but today I noticed the strangest thing.
We had just replaced a core component of the server for reading the socket connection from the browser. We ran a number of basic tests to ensure the new component worked correctly, then as a last test I tried to upload a large (1.6GB) movie file using a file upload control.
In IE, performance was apalling. On our 100Mbit network we were getting an average throughput of about 35Kb/sec. Naturally I assumed the new code had some performance limitiations so I started to profile it. The profiling showed no bottlenecks. Strange.
Next I fired up Firefox (with Pimpzilla theme naturally) and tried the same test. Amazing. 135Kb/sec! Same file, same everything. Problem solved (and an hour wasted).