Note how the [`ChromeActions.getPreferences` method](https://searchfox.org/mozilla-central/rev/4e8f62a231e71dc53eb50b6d74afca21d6b254e9/toolkit/components/pdfjs/content/PdfStreamConverter.sys.mjs#497-530) returns the preferences as a string, which we then have to convert back into an Object in the viewer.
Back when that code was originally written it wasn't possible to send Objects from the platform-code, however that's no longer the case and we should be able to (eventually) remove this unnecessary string-parsing now.
*Please note that in order to prevent breakage we'll need to land these changes in stages:*
- Land this patch in mozilla-central, as part of regular the PDF.js updates.
- Change the return type in the `ChromeActions.getPreferences` method, in a mozilla-central patch.
- Remove the string-handling from the `FirefoxPreferences._readFromStorage` method.
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