Chromium… I’m so getting downvoted with this one.
Why? Anyone is free to use whatever browser floats their boat 🤷♂️
Firefox itself is quite sluggish and slow to open on that piece of hardware
Do you get the same issues on an older version of Firefox for that device? If yes, proceed with caution - your device’s internal EMMC might be nearing EOL considering how old Android 6 is
But the problem is they all do not support modern arm64 apps that most Android phones use nowadays. Instead they need this other type called armeabi-v7a
They probably just stopped building for Android 6 devices. The SDK and various third party libraries continue to add new features all the time, and unlike Firefox, the majority of devs do not have the time or resources to manually code-in the missing bits to retain compatibility with old versions of Android. As a side effect, these custom implementations may have bugs or issues that go unnoticed due to the shrinking install base.
One of the more noticeable bits that changed is the Share API, which is why Firefox’s one looks so different vs every other app. There are other things like enhanced battery optimization and the storage API, which have changed a lot since Android 6.
IMO your best option is an older version of FF, or install Lineage (etc) on that device and use another browser
Edit: change “age of device” to “shrinking install base”
This infographic is really helpful. Stuff like this makes me relieved I use the majority of services in a browser, rather than native apps