I finally landed basic support for text selection in Fennec Native, and you can check it out in the latest Nightly build! Similarly to how text selection worked in XUL Fennec, you can long tap on some text to start a selection, and use draggable handles to change the selection area. A single tap inside the selection area will copy the selected text to your clipboard, and a single tap outside the selection area will cancel the selection.
There are definitely still a lot of known issues to work out, including full support for RTL pages, support for selection in inputs, and some problems with the handles. Text selection bugs are being marked as blocking bug 695173, so that’s where you can follow our progress. That’s also where you can file the bugs we haven’t found yet!