As a reminder, Firefox's Manifest v3 implementation will differ from Google's. Additionally, it isn't that Manifest v3 is the problem, it is that Manifest v2 compatibility is being mostly removed from all browsers. You can literally have V2 and V3 in your browser no issue technically there.

But it is important to remember what prompted this. Most extension authors are well meaning people, but in due course they will make decisions that aren't exactly widely accepted. Case in point was Stylish which was an extension that allowed one to apply various CSS styles to websites. Long story short, good intentions by the author, poor execution.

Manifest v3 was aimed to reduced the vectors that things like Stylish used to carryout their activities. This however, does damage the ability for uBlock to handle some of the various sophisticated ways adverts gets into your webpage. Firefox's implementation is an attempt to meet some middle ground. Additionally, Google has made some concessions to the final format of Manifest v3 to permit SOME ad-blocking that uBlock has indicated that they need.

In the end the important thing to remember is that uBlock Origin's manifest V2 version will be ending soon. There is a "lite" version of uBlock Origin that is compatible with Manifest v3, your mileage may vary with it however. Firefox offers the only solution at this time that is permissive enough to continue uBlock Origin's current development and ability to block most sophisticated adverts from getting into your page.

And the whole Manifest v3 issue highlights just how not open web standards are today. Yes, there is a standards body that issues the standards that govern the web. That body was once independent and sought a web for all. Today that standards body is mostly "what's Google doing?" And that has become problematic and mostly is why "everything is Chromium" at this point. Folks like Opera and Microsoft saw the political landscape at the Web Standards changing, read the writing on the wall, and converted over to Chromium. The vast majority of the issue is that the standards committee has become complicit in just accepting whatever Google offers up as a purposed standard. They have become mostly a rubber stamp for the machinations of Google's engineers to the detriment of everyone.