I did the same with my mom, but then her broker and doctor's websites started complaining that they will only run on a supported browser, i.e., Chrome.
I've gotten her to still use Firefox for most other things, but it's not easy.
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.
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