I don't know whether this would happen to you, but surely it happens to me quite a lot; let's say >5 times a day since Firefox download day. It was just closing itself or crash/freeze and needed restart the browser, esp. on particular web pages (Gmail and else) and more importantly on your tablet PC. There are a lot of Tablet PC user suffering from this issue, but then now solution is here now. Grab Firefox 3 nightly build--3.0.1-candidate, then this issue will be gone.
Some folks report this happen on one w/ Wacom Intuos3 as well. After update to this version, you will find yourself much more happy like you were with its predecessors.