Rewrite from scratch
Update highlight in right-hand menu when difficulty changes
Prevent user drag and select on non-puzzle elements
Add -moz prefixes to CSS
Use prefixes on box-sizing properties
Don't remove event listeners from nav buttons

They should only be added once anyway.
Fix box/flex rules in CSS for Firefox
Turn on select for input elements

Turning off select for all elements made it impossible to enter
high scores.
Use correct selector to make inputs selectable
Align puzzle without hbox

Firefox doesn't seem to render display:box correctly if there
is only one child inside the box. Use relative sizing and
margins to do centering instead.
Fix animations on slide menus
Fix vertical alignment inside slider menus for Chrome

The flex/box is a mess between Chrome and Firefox, and fixing
it for one breaks it for the other. Make it at least work
on Chrome until the mess can be resolved.
Clean up flex/box confusion for game and finish pages
hbox-flex is no longer necessary

hbox is now the same as hbox-flex, so remove all hbox-flex
instances.
Size background image correctly when puzzle is paused
Correctly add/remove event listeners to slide menus
Get rescaling of UI to work in Firefox

NB setting 'transform-origin' in JavaScript (e.g.
elt.style['transform-origin'] = 'top left') doesn't appear
to work in Firefox, so I pushed the rule into the CSS file.
Get photo chooser to display properly in Firefox
Prevent selecting elements on Firefox
Clean up lint errors