A user-agent spoofer like Chrome Mask is helpful here:

https://addons.mozilla.org/en-US/firefox/addon/chrome-mask/

It's off by default and has to be turned on per site, but will automatically stay on for subsequent visits. Test the site with it to be sure it works for your mom. Most sites that insist on enforcing Chrome-only user-agents run just fine on Firefox. I've encountered a few that run even better on Firefox than they do on Chrome.