Can't tap and hold on Firefox Android in text field to highlight, cut, paste, select all, share, etc

### Is this a hackmd.io issue?

- [x] Yes, the bug happens on https://hackmd.io
- [ ] No. If it's a self-hosted CodiMD instance, please fire up the issue in the repo of CodiMD: https://github.com/hackmdio/codimd

### What's the problem

#### Current behaviour

Can't tap and hold on my phone to paste text, highlight text, select all, cut, copy, etc.

#### Environment

I use HackMD on:
**Mobile**

- Device: Xiaomi Redmi Note 4X
- OS: Android 6.0
- Browser: Firefox v 68.0

#### Additional context

Add any other context about the problem here.

Example of this working as I write this:



This partially works in Chrome, allowing to paste and select all, but not to highlight, cut and copy.