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CloseReview for Theme Font & Size Changer for ThunderBird by Devott
Rated 5 out of 5 stars
For everyone out there not sure what to do with the xpi file downloaded via Happy One's link: I opened up Add-ons in Thunderbird, removed the old version, and then just dragged the new xpi file into the Add-ons box. Worked for me with ver. 52.5.2 on Win10
This review is for a previous version of the add-on (62.0).To create your own collections, you must have a Mozilla Add-ons account.