It is complicated. Here is the story of two WebView implementations in iOS:
Firefox is based on the WKWebView. Modern and Fast, but has a very poor (thin) API. It has zero options for interacting with the network stack, which is a requirement for content blocking. Bugs have been open since iOS 8, Apple does not seem to be interested or does not have the resources to surface more API. All the APIs are in WKWebView, just private.
Focus uses the older UIWebView. Slow, but better ability to interact with the network layer. Not ideal, but at least it is possible.
So to answer your question: We would have to replace WKWebView in Firefox for UIWebView. We've been very close to doing this in some form. But it is is big compromise in terms of functionality and stability. So I am not sure when and in what form that will happen.
There is a way to interact with the network latyer even when using WKWebView. Since iOS9 there are the Network Extension APIs which allow you to access (and modify/filter) raw packet data.
Yeah we know these things exist. I just don't think using a VPN or a global Network Extension or a global HTTP interception is a good solution. It would be a workaround.
We need a real solution. We need better WKWebView APIs. The requests are known. The bugs have been open since iOS 8. Two years.
That's pretty ridiculous for Apple, I understand they make it for safety purposes but they can't deny it also gives them an unfair software advantage. Kind of like old Microsoft and it's private undocumented APIs.
WKWebView executes out-of-process just like Safari and so it performs just as well. In fact, that's the whole reason that WKWebView exists (instead of just using UIWebView).
Is it planned to have content blockers support in Firefox for iOS ?