> On goes Firefox, uBlock Origin, and a couple other extensions. They don't care what browser they use.
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.
You can try Brave, it's Chromium under the hood so it's usually accepted and has built in ad blocking, and a one click way to turn it off for a specific site if you have an issue.
Did you try spoofing the User-Agent?
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.