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.