ok... took a couple of minutes to figure this out. thought i would post it in case others had a similar issue. i made several attempts to upgrade to firefox 3.0.10. each attempt did not upgrade cleanly. each time i then downgraded to 3.0.8. the last couple of times it only took a couple of seconds for gslapt to 'complete'. so i purged the cache and tried again. still no luck.
i decided to start looking into my add-ons. was gonna unload them one at a time to see if the problem was there. started with Download Helper. i clicked disable and restart firefox. upon restart firefox opened a bunch of windows. don't know why, but once i got all of those windows closed i retried the upgrade to 3.0.10.
viola - clean upgrade. complete with firefox telling me it has been upgraded. i don't know why, but it seems Download Helper was blocking the upgrade.
so for future reference - if your having trouble with upgrading firefox, disable the addons, particularly Download Helper. doesn't seem to be necessary to uninstall it, just disable. then retry the upgrade.