Firefox spelling check breaks on auto-complete pop-up

### Description
Firefox's in-built check spelling feature does not work correctly when writing a new message that includes something that causes the auto-complete pop-up to appear.

### Steps to reproduce
- Type an incorrectly spelt word in the "Send a message" field - observe that a red curly line appears below the word
- Keep typing a message and then try typing something that causes the auto-complete pop-up to appear (ie `:D`) - observe that the red curly line no longer appears below the incorrectly spelt word.

A work-around seems to be if you change to another Firefox tab, then change back again, the incorrectly spelt word is correctly highlighted.

I've notice this behaviour occurring as soon as I type a character like `:` and `#`. Interestingly If I use backspace to delete the `:` or `#` character and the space preceding it, the red curly line reappears below the incorrectly spelt word.

### Version information
- **Platform**: web
- **Browser**: Firefox v74.0.1
- **OS**: Linux (Debian testing)
- **URL**: riot.im/develop (b5e8e5cf7524-react-2929bcf9983c-js-63bc17a6b475)