A few years ago I thought it would be nice to be able to compose an email in a new tab instead of a separate window. I found a Bugzilla entry proposing adding this to Thunderbird that had been open for several years and had had a couple community members offer to address it, look into it for a while, and ultimately give up. The way Thunderbird's UI is implemented was just too intertwined with the code to let this happen without a major re-work. I feel like this kind of a change should be easy to do in a well-designed application and likely would be for something written with technologies.
I use Thunderbird every day and am generally happy with it, but often when I come across some feature that I wish were changed and start digging into Bugzilla it becomes apparent to me that the Thunderbird codebase is old, convoluted, and difficult to work with -- the result of growing organically over many years. I could see a lot of benefits to a re-write. On the other hand, trying to match feature parity with the current version would probably take a lot more work than the author of that proposal is accounting for.