Quill developer here. I'll look into the issue for Firefox for Android. It appears to be doing something different than its desktop counterpart. Feel free to submit an issue on Github for tracking though. It will be tough to officially support Firefox on Android though as we use Sauce for automation and it does not appear to be supported there.

Regular Firefox doesn't seem to be fully supported either. I've tried to select a line by double-clicking on it then clicking on the "B" button, but instead of making the line bold, it unselected it, selected the first character or word (depending on the line), and made it bold.

If I only select a few words it seems to work though.