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.

"Most sites that insist on enforcing Chrome-only user-agents run just fine on Firefox."

I have never encountered a site that requires a Chrome user-agent string

Some sites require a string that either (a) is in a list of acceptable strings (that includes non-Chrome strings) or (b) follows a certain format

FWIW, most websites submitted to HN, including many high traffic sites, do not require a user-agent header at all

What are some examples of these alleged "Chrome-only" sites