@LayLayL : I guess we will have to wait and see in the coming days and weeks when they have time to talk with the selected extension developers what level of Manifest V2 will be supported beyond the June 2025 cut off date in Opera based on your assessment. You are correct in clarifying that the only thing mentioned in the post specifically that will work beyond the June 2025 cutoff date is their own native ad blocking. It did also say in the post that Manifest v2 will continue to work until they decide the best path forward, so I have to assume that decision will need to be made well before the June 2025 cut off date.
This is not negate the fact that Opera is the only Chromium based browser besides Chrome with a add-on store which would make it possible to distribute add-ons that support some form of Manifest V2 beyond the June 2025 cut off date. If I am incorrect about this, please feel free to correct me.
At this point Firefox has only said they have "no plans to deprecate MV2 and will continue to support MV2 extensions for the foreseeable future", and that if down the road they change their mind they would give at least 12 months notice to developers. So it sounds like Firefox has a wait and see approach also, but it sounds like it will extend well past the June 2025 Chromium cut off date. Firefox does have its own add-on store so it does have the ability to continue to support Manifest V2 in the same way Opera could if they decide to do so. I don't think Firefox is the only real alternative when it comes to the Manifest deprecation until Opera makes themselves more clear as to what level of Manifest V2 they plan on supporting beyond June 2025 in their browser and add-on store.
With that said, I do think Firefox is a great alternative if you're looking for a browser engine other then Chromium, and you want something that is Open Source based. Firefox mobile is also nice if you require the same plug-ins that work on the desktop to run on the mobile version, but at the cost of performance. In many users experience (including my own) they choose a Chromium based browser because of better performance and security, and choose a Chrome based alternative like Opera due to the great included features.