Upgrading to the Release Channel is optional. You'll still get security updates and bug fixes if you don't update. Consider the following to help you decide. Release Channel: - less stable - newest features - other improvements, e.g., graphical/functional Note: some add-ons will not work until they are updated. Extended Support Release Channel (ESR): - more stable - consistent behavior - low maintenance - fewer new features See more details : https://support.mozilla.org/en-US/kb/choosing-thunderbird-release-channel/ https://support.mozilla.org/kb/thunderbird-release/