If you think Firefox is bad, try writing an extension for Thunderbird. All the docs talk about old XUL based extensions and new Jetpack ones, and there's even a

If you think Firefox is bad, try writing an extension for Thunderbird. All the docs talk about old XUL based extensions and new Jetpack ones, and there's even articles on their dev site about building Thunderbird extensions with Jetpack, but the truth is that Thunderbird support was removed from Jetpack. I had to check git logs to find this out! To get a restartless "Hello World" Thunderbird add-on running I had to bang my head against a wall for a solid week. It was not pleasant, to say the least. In the end the code itself wasn't bad (especially after I decided to just use Browserify and bypass their require() mechanisms completely), but finding the right incantations was horrific. The debugger situation was very bad, but looked like it was improving a little. I was able to get my Firefox beta to connect to my Thunderbird beta and remote debug. Except that it couldn't reliable set breakpoints, or step in and out of privileged code. And my console.log() message only went to the global debug console. Oh, and I had to write console.log()—extensions don't get that by default. Sadly in the end I discovered that the part I wanted to override was deep in the bowels of the C++ code, making it more or less impossible. It's an XPCOM component, so there's a small chance I could make a replacement component in Javascript and forward all but the functions I want to override through to the real one, but I got overwhelmed and stopped working on the project. :-(