We don't allow no stinkin' super dummies in here, so you do not qualify for that moniker.
I don't know if you can do what you want to do in FireFox.
If you are doing this in WordPress, and looking for those dimensions in your posts, your could use the Search and Replace plugin. That will scan the database and make all the changes in one pass. Just make sure you do a database backup first, just in case you screw something up.
WordPress › Search and Replace � WordPress Plugins