Minimize windows into the notification area for supported UXP applications.
Once you jumped through the hoops of getting a Firefox or Thunderbird set up for development, you can use a proxy file pointing to your cloned repository.
Most code is Javascript + some XUL and some CSS. But the low-level glue code had to be platform-native, meaning there is an dedicated implementation for Windows (a DLL) and one for linux (a shared object).
Please note that while the the Windows implementation uses C++, it does in fact not link the MS C/C++ runtime, so stuff like exceptions won't probably really work, and new/delete are implemented with calls to LocalAlloc. This is to avoid statically linking the massive runtime, and in fact, to avoid having two runtimes executing within the Firefox processs at the same time.
You need to have python-2 installed and then execute python build.py in the root directory of your clone. This will automatically generate an unsigned package.
Regarding the now-mandatory-in-Firefox add-on signing: You're on your own and have to figure that out yourself.