From Martin Brinkmann’s 30-May-2023 ghacks.net Mozilla Confirms Firefox’s End of Support for Windows 7, 8 and 8.1 Systems, which states in part:
“Mozilla plans to migrate Firefox Stable users on these three Windows versions to the Firefox ESR channel when Firefox 115 is released. This happens automatically and this special version of Firefox ESR will receive security updates until September 2024, its end of support date…This means that Firefox users on these operating systems won’t receive any new features implemented in Firefox after the release of Firefox 115, which Mozilla plans to release on July 4, 2023.
Mozilla has posted an FAQ about this migration to FF ESR v115 in the support article Firefox Users on Windows 7, 8 and 8.1 Moving to Extended Support Release.
When Mozilla ended support for Firefox on my Vista SP2 machine Win XP and Vista about 5 years ago I was automatically migrated from FF v51.0 to FF ESR v52.0 on 08-Mar-2017 and continued to receive security updates until support for FF ESR v52.9.0 (the final legacy Firefox browser released for Win XP and Vista) ended on 26-Jun-2018. As I recall, my migration to Firefox ESR v52 in March 2017 was completely seamless and did not affect my bookmarks, browser extensions, and other personal customizations.
Dell Inspiron 5584 * 64-bit Win 10 Pro v22H2 build 19045.2965 * Firefox v113.0.2 * Microsoft Defender v4.18.2304.8-1.1.20300.3 * Malwarebytes Premium v184.108.40.2068-1.0.2022 * Macrium Reflect Free v8.0.7279