[Firefox] Entering 'à' results in 'aà'

### Expected
When using the method depicted below, only one letter should be added to the editor.

### Observed
Two letters are added: the 'base' letter, and the one with diacritics.

https://user-images.githubusercontent.com/73365754/107675495-cfb14300-6ca0-11eb-8ef1-4a5a3119b88f.mov

### Reproduced
1. Use Firefox on Mac
2. Create new note
3. Press and hold 'e' key, for example
4. Once a popup with diacritics appear, select the option ~> this will result in two letters: 'e' followed by the one you've chosen.

### Where did you see the bug
- System Model: MBP 16"
- OS: macOS 11.2
- Browser: Firefox 85.0.2 (64-bit)
- Simplenote app version: 2.7.0-beta1

Reproduce the issue in Firefox on macOS by creating a note and choosing a diacritic from the held-key popup in the editor. Trace the editor's input handling around that interaction; done means selecting a diacritic inserts only the chosen letter, not the base letter followed by it.

Written by the indexing model from the issue text.