Clicks twice on the submission image, scrolling to the image and scaling it to fit in view.
Update, I've never managed to get this to work consistently but have found an extension for FireFox that works a lot better: https://raccoony.thornvalley.com/ I'll leave this script up for anyone who needs it and can't use the Raccoony extension.
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Fit the image preview to the window by default.
It works in a really dumb way - waits a moment and then clicks twice on the image. You might need to adjust the timeout on slow systems or network connections. Possibly you need to have it click a different number of times if your IB account is set to something different for the default view?
Let me know if you have better timing for when to click - ideally I'd wait for some HTML event. Clicking too early in FF makes it think you want to pop open a new window of something.