From Firefox support for Windows 7, 8 and 8.1 (https://support.mozilla.org/en-US/kb/firefox-users-windows-7-8-and-81-moving-extended-support)
QuoteFirefox version (https://support.mozilla.org/en-US/kb/find-what-version-firefox-you-are-using) 115 is the last supported Firefox version for users of Windows 7, Windows 8 and Windows 8.1. If you have been using Firefox on these versions of Windows, you will be moved to the Firefox Extended Support Release (https://www.firefox.com/browsers/enterprise) (ESR) channel by an application update.
What is changing?
- Firefox 115 is now the last version supported on Windows 7, 8 and 8.1.
- Updates will be delivered through the ESR channel until the end of August 2026. It was previously announced that support would end in February 2026, but in order to ensure our users can continue to browse the web, we will extend support for Firefox for these operating systems for another six months and then re-evaluate.
- After that, if support is not extended, users should upgrade their operating system to continue receiving Firefox security and feature updates.
Additional information in the referenced article.