Rebase Fenix patches to 81.1.0b1

We have a first Fenix beta tag we likely want to use to resolve the remaining build issues and get our nightly builds out.

I pushed a branch, tor-browser-81.1.0b1-10.0-1, to the fenix repo which corresponds to the official v81.1.0-beta.1 tag.

Now we need to rebase our patches onto that branch.