Thank you for the review and the updated comments. When this was a legacy addon, I was able to add zoom support. However, when we moved to webextensions, I was unable to restore the original zoom settings. This is more likely because of my own limited understanding of how to do this in webextensions. I will revisit this problem and see if I can find a solution.

Update 2019-12-11:
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Mozilla has refused to implement webextensions API's that can support custom zoom restoration without the extension developers having to write a lot of complicated code. Please refer: https://bugzilla.mozilla.org/show_bug.cgi?id=1588580

Update 2020-08-31:
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Version 3.0.21 (available from August 31, 2020) tries to address this through a work-aorund. I scale the font size based on the zoom percentage rather than reset the zoom to 1 before applying the extension specified font size.

There may be a slight difference in the font size of zoomed pages (+/- 1 px; compared to the extension customized font size) due to rounding.