I definitely recommend updating to Firefox-ESR 115, but I just wanted to point out that this warning on most websites is usually caused by your user-agent. You could probably change your user-agent using the build-in about:config page or by using an extension and continue using all sites normally.
This is the current user-agent for Firefox-ESR 115 and the Tor Browser (both using the privacy.resistFingerprinting setting):
Mozilla/5.0 (Windows NT 10.0; rv:109.0) Gecko/20100101 Firefox/115.0
But again, I recommend updating to Firefox-ESR 115.
And as for my Mobile-Friendly Firefox Customizations for Librem 5, I recommend reading the very first post in that thread if you aren’t at all familiar with enabling CSS code for Firefox. Don’t save any files from that post, just read it.
After that, you should start with October 2023 Halloween Update (post 55). The final post in the October 2023 Halloween Update multi-post is Styles & Screenshots (post 61).