Amazing review haha, thanks! :D I'm glad your retinas aren't burning anymore!

Regarding your points:

1) I'll add a new style option in a future update for the profile page comment background colour.

2) Unfortunately, I can't make the popup stay open, but I have a few options you can try:

a) I added a "shade" button at the top right of the popup window. This will "minimise" the popup so you can see underneath, but if you click outside, it will close.

b) Perhaps a better way, you can click the "Options Page" button to open the options in a new tab. You could then drag and detach the tab and put it on a second monitor, or resize it and switch between it and your main browser window, or just keep it as a tab.

c) Bonus option. Go to "about:config" (in the URL bar) and search for "ui.popup.disable_autohide" and toggle it to "true". This will keep the popup permanently open, but you can't close it unless you toggle the option back to "false". You could combine this with the shade option!

3) I'm afraid not, I can't access user local storage for security reasons. However, it is possible to convert images to base64 code, then that code can be stored safely in the extension local storage in the browser. I think the maximum an extension can store locally is 10MB, so you'd have to use a small image or reduce the file size. Browser sync storage is only 100KB, so syncing images is out of the question. I could automate this process and you simply just upload an image, but you won't be any to store many.