If you were as resource-constrained as Mozilla is, you'd drop support for unsupported platforms anyway.

According to this they still support windows 7 through extended support channel: https://support.mozilla.org/en-US/kb/firefox-users-windows-7...

Chrome dropped support two years ago or so.

"Firefox version 115 is the last supported Firefox version for users of Windows 7, Windows 8 and Windows 8.1."

That's a version from over 2 years ago.

ESR version doesn't mean it has never received changes for 2 years.

The problem is not the lack of patches. The problem is with websites refusing service based on client's version from the user-agent or breaking by using cutting edge features without a polyfill.