I updated FF to version 3.6.3 a week or two ago.
Any idea why I am now getting a pop-up above the clock asking me to up-date to ...version 3.6.3 every time I log on?
A last resort solution would be to save your user profile (either with a utility like MozBackup or by copying the appropriate folder from your profile in Documents And Settings) followed by complete uninstallation using e.g. Revo, then reinstall Firefox afresh and finally restore the user profile. But that would be a very last resort option should no other better solution be suggested.
I usually work on the principle of "If it ain't broke, don't fix it". So I've disabled the automatic update feature of Firefox. Doing so might also solve your problem:
Go to Tools>Options>Updates . Click to remove the relevant tick(s) and then on 'OK'.