You can use the plain text file user.js in the Firefox profile folder to initialize prefs on startup. * user_pref("devtools.netmonitor.requestfilter", ""); The user.js file is read each time Firefox is started and initializes preferences to the value specified in this file, so preferences set via user.js can only be changed temporarily for the current session. *https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save You can use the button in "Help -> More Troubleshooting Information" (about:support) to go to the current Firefox profile folder or use the <b>about:profiles</b> page (Root directory). *Help -> More Troubleshooting Information -> Profile Folder/Directory:<br>Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder *https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data