Firefox, Opera and Ubuntu 9.04 Jaunty

Here's a weird thing.  A couple of weeks ago, my Vaio notebook that's been running sequentially updated versions of Ubuntu GNU/Linux for the last 2.5 years started playing up on me with apparently random lock-ups.  This didn't start after any specific event or software installation.  I've Googled this, looked in Ubuntu Forums (and indeed started a thread there) to find an answer.  The symptoms are:

1. Display frozen, except the mouse/trackpad pointer can be moved
2. No keyboard input
3. No trackpad buttons work
4. The ongoing processes are still running - I can login from another terminal

This always seems to happen when I'm using Firefox (I am currently on 3.5.1), which is by far my favourite web browser, though this might just reflect both the amount of activity when using that app as well as the number of times I use Firefox each day.  Often the hang occurs when I'm scrolling down a page (which I do by stroking the edge of the trackpad).

I have found that the hangs occur in both Gnome (my preferred window manager) and KDE.  I have yet to turn desktop effects off and try that.  Any hints as to how to divine what's going on would be most welcome.  Nothing I've seen in the system logs are of a great deal of help unfortunately.

My current strategy is to switch to my second favourite browser, Opera.  I've been irritated by Opera's lousy font rendering, and starting to use it again I felt impelled to try and find a solution.  Google turned up this page, which has a really easy Preferences tweak that makes the font rendering much more satisfactory.