I understand the "WebKit wrapper" for iOS criticism and do want different rendering engines available (e.g. Firefox's) and yet...
The worse thing about Firefox for iOS is the wrapper part, not the lack of rendering engine choice part. The UI of Firefox on iOS is inconsistent and buggy, and syncing doesn't sync well, etc. I doubt using FF's own rendering engine instead of WebKit would help the situation, as it'd drain engineering resources away from making the "wrapper" more usable.
I've always used Firefox, on Mac, Linux, Windows, Android, everywhere I can, but I find myself using it less and less on iOS... and it's not because of the rendering engine!
But since this change for the DMA will allow FF to use its own rendering engine (in the EU), hopefully maybe it'll reenergize the development of FF and improve the "wrapper" part more - even for non EU users!