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"

Last time I checked ARM v7 is not the issue, there are still up to date builds available from Chrome itsself or Brave, rather Android 6 is. Google seems to have a cycle where roughly every fall they drop another Android version.

Right now the minium requirement is Anroid 8 and if the cycle continues it will loose support in a few months and Android 9 will be the new minimum requirement.

However I also have a a few Android 6 or 7 devices and usually firefox runs fine on them if they at least have two proper large CPU cores. But using two year old Chromium based browsers, I never ran into any sites that wouldn't work correctly.

First buy a new phone.

Nah, just kidding 😂 maybe you should check Cromite (Bromite fork) https://github.com/uazo/cromite/releases.

If Bromite supported old devices maybe Cromite does the same?

But for sure this one from DivestOS will work https://divestos.org/fdroid/official/Mulch-arm-126.0.6478.122-1.apk (Chromium version of Mull)

https://github.com/Alex313031/Thorium-Android There is this browser it supports arm32, I don't see support for armeabi v7a, I don't know will it work, not really familiar with tech side of that

https://www.viayoo.com/en/ This is browser for older devices, very lightweight, even managed to install older version on Android 2.3.4

https://f-droid.org/packages/org.mozilla.fennec_fdroid/ And this is Firefox fork, maybe it will run more smoothly than Firefox

But what about my embedded ad-blocker?

Downvoted bcos Op bi+ched about downvoting in the beginning of text, that's bad behaviour, do better

Although, the Chromium project just like Android are free, they're stained by being majorly developed by google and are the major source of spying and tracking today

FFUpdater supports Android 5+: https://f-droid.org/packages/de.marmaro.krt.ffupdater/ You can install a lot of browsers from there, maybe one of them supports this phone: https://github.com/Tobi823/ffupdater

Also check slimsocial, it's a facebook client, supports Android 4.4+ and it lists arm7 support: https://f-droid.org/hu/packages/it.rignanese.leo.slimfacebook/

And new Chinese phones (Redmi, HMD, etc) are really cheap, they are good as a backup smartphone. I know it sounds terrible, but you can't really do anything with planned obsolescence. Our time is short on this planet, waiting for websites on old hardware simply doesn't worth it, you should spend this time on more important things.