So I was working on the charting plugins new search request view popup where, when we popup the chart, we try to be all sexy and fade the background to a slightly transparent black that makes the popup look very modal. The good news is that it looks very swank, the bad news is that on my machine every time I click it my CPU usage jumps to about 50% and my machine become unresponsive. I know sometimes there is a stability/performance trade off for good looks, just ask Jens with his whining mac book, but this was ridiculous.
It turns out that only on Firefox 2 under linux is this a problem. The problem lies with the css attributes -moz-opacity: 0.8; opacity:.80; filter: alpha(opacity=80);. These are the bits that produces the magic slightly transparent background. It seems that somewhere in the transition from Firefox 1.5 to 2 the rendering of this, under linux, has gone terribly wrong. It might be the case that some magic rpm or deb is just lurking around the corner to fix this up but it was broken for me on Fedora Core 5 and for Nick using Ubutu (and the stock debs).
For the time being we have put a hack in place that will check to see if the user is using Firefox 2 on Linux and if so the background will be the grayish color, but not transparent. This fixes things up.
Once again, twice within a week, I am reminded that if you get a support case that seems impossible and you just can’t reproduce it, it really pays to try to replicate the environment of the user as close as you can, browser versions and everything.

Opacity css seems to kill performance in Firefox 2 under Linux