Extremely poorly tested updates. I'm running fresh Windows 10 (fully updated), the newest stable release of Firefox, and even with the browser uninstalled, then user profile completely removed by deleting C:\Users\[user_name]\AppData\Roaming\Mozilla and then the browser reinstalled again (so no other extensions could possibly get into the way of this extension) 18.0 still didn't work properly (after reopening the browser the extension displays the previously set zoom levels on the toolbar icon but these zoom settings don't actually get applied to any website).

Find another extension if you need reliability or at the very least turn off updates for this extension.