Hello,
Per canonical/firefox-snap#53, we are transitioning the Firefox snap from using a mount-control interface for hunspell system dictionaries to using a system-files interface instead, a simpler alternative that per snapd folks should also fix issues like LP#2059195 during installation/upgrade of the Firefox snap. We therefore hereby request approval for auto-connection of the firefox:host-usr-share-hunspell plug of the system-files interface please.
For my preliminary review and testing of the change, I’ve only applied it to the beta and nightly branches – which correspond to higher-risk channels – and I plan to test a few more installation and upgrade scenarios after the approval of the request, before finally applying the change to stable as well.
The Firefox snap has existing approval for auto-connection of firefox:host-hunspell with the mount-control interface, which I hope combined with the fact with this change we’re aiming to move towards a simpler, better, and more streamlined approach would make for a smooth review process. Please do let me know if you require any more details/clarifications for this request, though. 
Thank you,
-bandali