This kb article explains the March 15 date. https://support.mozilla.org/en-US/kb/root-certificate-expirationhttps://support.mozilla.org/en-US/kb/root-certificate-expiration You need Firefox 128 or later or if using the older Firefox 115 ESR you need at minimum 115.'''13'''.0esr to not be affected. The main change you may notice is any extensions you have installed in Firefox may not work after the date if you continue to use the old Fx 115.12.0esr. Firefox 115 ESR is currently at 115.21.0esr and will be updated til Fx 115.27.0esr in August unless Mozilla extends updates again for third time. If there is no option to check for internal Firefox updates in your Firefox 115.12.0esr install then it may likely be a package build and not a official build from Mozilla. Mozilla only provides internal Firefox updates to their own Windows, macOS, Linux builds from mozilla.org Debian Buster was made EOL on June 30, 2024 which would explain why you have the old Firefox 115.12.0esr (released June 11, 2024) as you no longer get anymore Firefox package updates. https://www.mozilla.org/firefox/releases/