Many versions back, forget how many, we did have a working version of Enigmail, unluckily Mozilla changed its rules about addons and I didn't/don't have the skills to port it.
The problem was that Enigmail used a binary (DLL) and then changed to calling system calls through CTYPES, which meant calling the OS/2 API (libc would have worked too) from JavaScript manually, ie setting up the stack etc to call OS/2 functions to read and write files and such.
No idea of its current status since Mozilla has changed to WebExtensions.