It's the same as the situation with Chrome/Chromium. There are a million "de-Googled"/"privacy focused" alternatives to Chrome all using the same engine, and when Google pushed manifest v3 changes to block ad-blockers every single one of them was affected.
You are making an orthogonal point. Yes, Google maintains AOSP. No, that does not mean that AOSP OSes that are not in Google's Android program (calling it that to avoid semantics games) have to adopt this change. If you want to hear it from the experts: https://grapheneos.social/@GrapheneOS/116103732687045013
At any rate, this particular Google anti-feature does not require a large patch (or maybe none at all).
That's just objectively wrong, both Brave and Opera still support manifest v2 and are committed to continue doing so for the foreseeable future. Even Edge apparently still has it, funnily enough.
Brave does NOT support manifest v2. They have instead hand picked exactly 4 manifest v2 extensions (AdGuard, NoScript, uBlock Origin, and uMatrix) and have hard-coded special support for them. They quite literally say in https://brave.com/blog/brave-shields-manifest-v3/ that all other v2 extensions will go away from Brave once Google fully removes support for them (which may have happened already, since it was posted a while ago).
As for Opera (https://blogs.opera.com/news/2025/09/mv2-extensions-opera/):
> MV3 extensions are the new standard and will offer a more stable and secure experience. Opera itself will shift to an MV3-only extension store.
You're misreading that page, they have special cased the hosting of those 4 extensions, because they do not have their own addon web store and are relying on Chrome's instead. You can still install any manifest v2 addon manually, not that there are going to be many outside of those 4 that care about v2.
As for Opera:
"Today, we reiterate what we said back in October 2024: MV2 extensions are still available to use on Opera, and we are actively working to keep it that way for as long as it’s technically reasonable."
Read: for as long as Chromium allows this via a flag.
addons for firefox were at first a way to test features. we only have devtookls because one person wrote an addon copying ie6 dev tool. next Firefox release it was part of the core browser.