Amazon released a new firmware (5.8.11) for their Kindle devices (6th generation and up). The firmware update is a significant one, as it aims to provide extra typographical features to their e-readers (a weak point with Amazon’s Kindle firmware).
Despite some improvements, significant problems persist. First, the option to increase font weight only works with AZW3 file, so if you have a library of MOBI e-books the feature does not work. Further, Amazon, oddly, does not support sending documents to your Kindle e-mail in the AZW3 format. This means the only way to get an AZW3 on a Kindle is to sideload the document but this means the e-book is not archived in the Amazon cloud and synced across devices. Second, font size selection is still poor and the increase between different sizes - in the new font scale - remains disproportionate. It would have been better if the firmware allowed the user to select an exact number for font size.
Amazon, overall, offers a stable and feature-rich firmware experience, in comparison to other vendors. However, in the case of typographical support, they remain behind Kobo. Yes, Kobo also has issues with font size but, at least, there is more flexibility in size selection. Further, they allow the user to increase font weight in both EPUB and KEPUB e-books (the feature only works with Kobo designated fonts and not side-loaded ones).