@joginder89 to solve Windows 7 issues you have several options:
1.- Use an updated Firefox (or at least version 50) as far as I know ISRG Root X1 certificate was added to Firefox in that version.
2.- For some reason Windows 7 is not lazy-loading ISRG Root X1 certificate, maybe because you have turned off updates (even whether Windows 7 is not receiving more updates, it receives them for Trust Stores)... or for whatever reason, so you could try to install it manually, you could use this reg file to do it easier Fixing Windows installs that don't receive updates to their trusted roots - #29 by rmbolger
3.- I don't like this one but you could use another CA to obtain your certificates.
Cheers,
sahsanu