Update all plugins, uninstall any plugins you no longer need or disable all plugins, to see if that resolves the problem.
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You will not be able to start the browser until you first end all browser processes or restart the computer.
(If this is a recurring problem, read this for other possible solutions.)   Try starting Firefox or Sea Monkey in Safe Mode (see the linked article for details).
These troubleshooting steps get progressively more difficult and time consuming, so we strongly recommend attempting them in ascending order to avoid unnecessary time and effort.
Manually editing the Windows registry to remove invalid Error 0x C80003F3 keys is not recommended unless you are PC service professional.