I think your request is misplaced. You are using, what did you say, gmvault to create your archives. I suggest you approach them, it is an open source project, for some way to actually interact with the emails. Thunderbird has it's own archive feature and any developer effort will be placed in making it better. Not in making a user interface for a gmail archival tool.

I think you've misunderstood or misinterpreted my question... .eml files are not specific to gmvault, and compressing .eml files isn't either. With several years worth of what are basically text files, compression is quite useful. So why is asking for a feature like this misplaced? It's fair enough to say it's not a priority, but it's in the right place... Your response seems quite dismissive.

And where did I ask for a UI for gmvault? I would like to use Thunderbird to read / search .eml files that are compressed. Is that better?

EML files are text files. Not compressed in any way. Thunderbird will open them natively right now. What it will not do is work with compressed files created by your chosen software. It might be nice for those using the software to have a way to use it with Thunderbird. But that is something that software developer would provide. Hence my view your request is misplaced.

While the English language uses the word compressed to discuss file compression generally, you are talking about specific algorithms (Gzip) and implementations. In some parts specific to your chosen product gmvault, such as the JSON parts of your topic.

Based on your initial request I had to infer from the information that you supplied that the files used the GZip algorithm. You then went further and wanted the application to do something with JSON files that were somehow entangled in your use of the term compression. There are probably a dozen popular in use compression algorithms, so you request as it stands is both specific and requiring implementation of support for gmvault and the storage formats it uses for files.

An I dismissive. No, I think it would be a great addition to the gmvault offering. Do I think it stands a snow balls chance of being supported by Thunderbird's core developers given is will benefit a very very small percentage of Thunderbird user base. No I do not. But I do not make such decisions, just make educated guesses. You are however more than welcome to share your idea in the feedback forum, there is a link on Thunderbird's help menu for that purpose.

If this forum isn't the right place for a question of the form "is there a setting or add-on for X" where X is something out of the ordinary, then that wasn't clear to me, and apologies for the disruption. Your opening line of "your question is misplaced" seems to suggest there are some rules here I wasn't aware of.

Thunderbird can open individual .eml files of course; I was hoping for a way to 'browse' folders of thousands of them, which I don't believe it can. The fact they are compressed makes it even more challenging, unless Thunderbird could (through, e.g., an add-on) read such compressed files.

In any case, I also asked over at the Betterbird forum, and the response there was that "although the underlying application can't do this directly, here is a method that might work", and their suggestion is working very well for me now, so all good.