Firefox 137.0 is a Release while Firefox 128.9.0esr is a Extended Support Release or ESR. The ESR meant more for Enterprise and such users in mind while regular Firefox users use the Release. The Firefox 128 ESR is based on the Firefox 128.0 Release but with security/stability fixes since as Enterprise users like longer term stability. https://support.mozilla.org/en-US/kb/switch-to-firefox-extended-support-release-esr https://www.mozilla.org/firefox/enterprise/ Currently the older Firefox 115 ESR channel is still being updated so the eol Windows 7, 8, 8.1 and macOS 10.12, 10.13, 10.14 OS users can have a secure version of Firefox still though the last planned update will be Fx 115.27.0esr in August unless Mozilla extends updates again.