Firefox will work out of the box.
However, Chrome should work just fine on Win 7. If Chrome has issues, this is an indication that the OS (Windows) has not loaded ISRG Root X1 for some reason. This should have been done automatically for Windows users.
For systems where this failed, it is possible to install ISRG Root X1 manually. Once that is done, Chrome should work again (with all Let's Encrypt certified sites).