Vladimir Vukićević pointed me to an old blog entry where he provides a useful "bookmarklet" that I thought I'd share. It makes it easy to measure how long mozilla spends rendering any particular web page.
My version is slightly tweaked from Vladimir's and is available here: Time Render.
The differences in mine are that it runs for 300 iterations by default instead of just 10, (long enough to collect very healthy profile information), and that it emits its result by dumping to stdout or stderr instead of popping up an alert box. Not only is the alert box hard to use when it has 300 values laid out without line-wrapping, but I do X server experiments where I need to measure performance even when things are rendering incorrectly, (so if the results are only displayed on the X server they are useless to me).
To use this, first you'll need a recent Firefox, (I'm using a daily
build here which works, where the system 2.0 version does not seem
to). Just drag-and-drop the above link to the bookmark toolbar, (and
yes, I had to turn it on just for that). You might also need to go to
the "about:config" location in the toolbar and create a new Boolean
entry, (right-click to find that), named
browser.dom.window.dump.enabled and set it to True.
At that point you should be all setup. Just browse to a web page of interest and click the "Time Render" bookmark to measure the rendering performance of Mozilla on that page.