How can I download version 115 for my W7 computer

You can download it from https://www.firefox.com/en-US/download/all/desktop-esr/win64/en-US/

Or if you need Firefox for 32-bit Windows then see https://www.firefox.com/download/all/desktop-esr/win/ as you pick language then click on the [Download ESR 115] to get 115.31.0esr. Firefox 116.0 and later requires Windows 10, 11 to run. The last planned update for the older Firefox 115 ESR will be Fx 115.33.0esr unless Mozilla decides to extend updates yet again.