You mentioned using a few distros before. Did this error occour already in the first days/ weeks of using the new install, or did it occour in the long term?
If that’s the case, consider switching to an image based (“immutable”) distro like Fedora Silverblue. Due to their image-based nature, they don’t have any package drift.
On a traditional system, the configuration slowly changes over time, due to updates and new installs, and therefore bugs may occur that only you have, so they’re not fixable.
An immutable system is reproducible and should contain less bugs.
Maybe try that?
Have you deleted Firefox, removed all data and reinstalled it? Probably won’t help, but it might?
Does it only happen on this device, or on others too? Maybe the RAM is faulty. Do you also have other freezes and crashes, like when you’re gaming?
I had similar issues when I had a damaged/ misconfigured RAM.
Or your storage (SSD?) is faulty?
Is it only on Flatpaks, or on native packages too?
A small bandaid workaround would be to just stop using Flatpaks and use Distrobox containers. They work fine too.
But I would try to fix the underlying issue first if I were you. I think it’s a hardware problem.