Changes
→Setting and Locking Preferences
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You may choose to use obscured values in mozilla.cfg by changing the obscure_value above to 113 (for byteshift 13), but beware you will need to go through an additional step to convert the plain-text config to an obscured version every time you make a change. Also note that the method used to obscure the config file is easily reversible.
Next, create your mozilla.cfg file. Open Firefox and enter about:config in the address bar to see all of the available configuration options. Note that the first line must a comment, any options configured in the first line will be ignored.