They do work on it. A lot.

But the issue is browsers don't make money. You can't charge for it, you can't add ads to it, etc. You're competing with the biggest companies in the world (Google, Apple), all of whom are happy to subsidize a browser for other reasons.

I might be in the minority here, but I actually like Thunderbird.

I assume they work on Firefox 10x more than anything else. Is there data?

>Firefox - the one thing they do not want to work on

I'm sorry but this is complete nonsense. Just this year they pushed 12 major releases, with thousands of patches, including WebGPU efficiency improvements, updated PDF engine, numerous security fixes, amounting to millions of lines of new code. They maintain a codebase that rivals that of Chrome and of the Linux Kernel and push the equivalent of Rust's entire codebase on a monthly basis.