Earlier quoted context omitted.
You can use Firefox Developer Edition and make your own addon. It isn't exactly straightforward, especially the "communicate with external software" part (e.g. in Windows you need to put some entry in the registry to let Firefox know what program you need to communicate with) and communication has to be via JSON which in practice means you need to write some sort of proxy app. But it is possible. I have made my own l…
Any specific reason to use FF dev edition? You can use your own addon with regular FF by signing it with web-ext CLI tool.
I do not like the whole signing thing - just like i do not need to sign any -say- Emacs scripts i write, i shouldn't need to sign any browser scripts. Also last time i checked i need to register with Mozilla for this to work and i especially do not want to have some 3rd party's permission so that i can install my own scripts on my own browser. I might have misunderstood that last part but i didn't see much of a reason to bother when i could jus ignore the entire thing with the Developer Edition.
Finally this seems to need npm, node.js, etc to work and i do not want to bother with these things. All i did to make my addon was to use a text editor, make 2-3 files and done. Well, and use regedit but whatever...