I had a similar issue on one of my android phones and in my case, switching to
Bromite stopped the freezing. Which is odd in a way, since it's just a stock Chromium browser with some features both added and stripped out of the base source code. It may not work, since its Chromium-based, which seems to be the issue for your phone.
But you are not the only one with this issue. Quite a few of the Redmi note (Pro) smartphones have had this issue for a year now. Shared symptomes being stuttering, freezes, terrible optimization (or lack of it), if not just not working at all in regard to Chromium-based browsers. It was spread over several Xiaomi models from which Redmi suffered the most of.
One of the solutions I know of,
disclaimer: may not work for all models or situations!, was to either reinstall or uninstall the android system app called "Android Web View" using the Google Play Store.
Play Store -> Installed Apps -> Search for Android System Webview
This system app in short is an embedded browser used to for example display html emails, render web based apps, etc.
Note: Doing this has a small chance of breaking browser-based apps, if you use any! Always test and revert back when problems arise. Which subsequently means using Firefox in your case after reinstalling the system app back.
It has something to do with Xiaomi's backend in the android rom, if I remember correctly. Since that system app is used for more than just the browser and somehow conflicted with it somewhere.