Firefox version 115 is the last supported version for users of macOS 10.13 (High Sierra).

If you visit the Firefox download page at https://www.firefox.com/ while using macOS 10.12 (Sierra), macOS 10.13 (High Sierra) or macOS 10.14 (Mojave), you will be prompted to download Firefox Extended Support Release (ESR) to install the latest Firefox 115 ESR version.

Firefox users on macOS 10.12, 10.13 and 10.14 moving to Extended Support Release

Firefox 116.0 and later including the current Firefox 140.6.0esr and 146.0.1 Release requires macOS 10.15 (ten.fifteen) at minimum to run.

However if the macOS 10.13.6 is the most current macOS you can update to then Firefox 115 ESR (115.31.0esr) is the most current Firefox you can run.