I found a BrushedMetal-XP.persona.json backup file, how do I install it?

Thunderbird prior to version 115 is not supported. Versions prior to 68 when XUL addons were largely deprecated are nothing but a memory of the peers that volunteer their time here. Versions prior to 68 also have issues with modern security and connections to major mail provides no longer work.

As the site addons.thunderbird.net is the official and only site that has Thunderbird addons associated with the Thunderbird project, you can find a copy there or no one will be able to help you. Perhaps one of these search results https://addons.thunderbird.net/EN-us/thunderbird/search/?q=brushed_metal&cat=themes

I do not recall any addons that existed as a JSON file, they were, and still are, distributed as renamed zip files (XPI) that contain a lot more than a JSON file.

So I will respect your decision to use unsafe and obsolete software that was never released (the release version went from 52 to 60. There were only beta versions in the 58 series) , but please do not ask for support from peers in a peer support forum such as this as you basically have no peers on version 58.

Thanks for the detailed answer. I already tried these links, there is no old version of this theme.

I meant 52.8.0, not 58, my mistake.

This version works with all my providers, including gmail despite their new auth method. You just need to generate an app password, absolutely nothing unsafe about that.

You can do stuff with XUL than you cannot do with WEB-EXT, period. WEB is standard now and easier to maintain, that's better for Modzilla, not for the users.

I tried Betterbird, since Thunderbird now has what? 1000, 2000 bugs that Modzilla is too lazy to correct?

If I have to let go of my old Thunderbird, I'll use that. It's the nicest mail client I tested for power users, still comes a little short in convenience compared to 52.8.0 with all the addons.

json files were generated by TEBE.

And about not asking questions here, this kind of forum is for 2 different kind of users:

- Newbies that need help with standard stuff

- Power users that need help with something .01% of people use.