They have. It's called bypass-paywalls-clean . It works pretty ok.

It just keeps getting banned from the addon catalogs because of complaints from media. The Firefox one was taken down by a french newspaper. So you have to sideload it, which is hard to do on Android.

Edit: it looks like even the github was taken down now: https://github.com/iamadamdev/bypass-paywalls-firefox

But yes it exists. And it works for most sites. It's just hard to get it now.