Pinned tabs are stored in sessionstore.jsonlz4 as part of the session data. Make sure you do not run Firefox in permanent Private Browsing mode (Always use Private Browsing mode; Never Remember History). *https://support.mozilla.org/en-US/kb/private-browsing-use-firefox-without-history *https://support.mozilla.org/en-US/kb/common-myths-about-private-browsing *Options/Preferences -> Privacy & Security<br>Firefox will: "Use custom settings for history" *remove checkmark: [ ] "Always use Private Browsing mode" In case you use "Clear history when Firefox closes" to clear history. *do not clear the Browsing History *Options/Preferences -> Privacy & Security<br>Firefox will: "Use custom settings for history":<br>[X] "Clear history when Firefox closes" -> Settings *https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox *clearing "Site Preferences" clears exceptions for cookies, images, pop-up windows, and software installation and exception for password and other website specific data *clearing "Cookies" will remove all selected cookies including cookies with an allow exception you may want to keep

This is a browser bug. You need to make a more reliable mechanism for remembering pinned tabs.

Maybe this extension helps: *StartupApps https://addons.mozilla.org/firefox/addon/startupapps/