Submit extension to Firefox store

[jeffbl]

When going to the chrome web store link for the IMAGE extension while using Firefox, it just tells you to switch to Chrome. My understanding is that we have to submit our built ZIP file to the Firefox add-on store, but I'm not at all familiar with the requirements:

 https://addons.mozilla.org/en-CA/firefox/extensions/

Goal is to have the ability to easily load IMAGE extension into Firefox, as we currently have for Chrome.

[jeffbl]

Although I'd love to have Firefox support sooner rather than later, I'm slipping this out to larger beta, since @Clarisa1999 you have many other more pressing items on your plate for UI, etc.

[jeffbl]

This is pending a fix for the button not showing up for some charts.

[jeffbl]

Postponing a sprint to give more time to charts, since it looks like this may be nontrivial.