Mozilla is no longer serving the older Firefox 115 ESR on https://www.mozilla.org/firefox/all/desktop-esr/ since the Fx 115.16.0esr update so you will have to either go to https://www.mozilla.org/firefox/windows/ or use https://archive.mozilla.org/pub/firefox/releases/115.16.1esr/ to download the current Fx 115.16.1esr.

The Fx 115 ESR updates are for Windows 7, 8, 8.1 and macOS 10.12, 10.13, 10.14 OS users in mind now as Windows 10/11 and macOS 10.15+ users will get update prompts to Fx 128 ESR instead of continuing with Firefox 115 ESR updates.

There is a copy at this address – https://archive.mozilla.org/pub/firefox/releases/ (Effectively, the same link as the third one above.)

At the moment the current Firefox 115 ESR is Fx 115.18.0esr and will be updated til at least Fx 115.21.0esr in March 2025 unless Mozilla decides in early 2025 to extend updates again.

https://archive.mozilla.org/pub/firefox/releases/115.18.0esr/