Previously, I could write an addon for personal usage packed as something.xpi
and I clicked on it to install it.
After a while, mozilla introduced xpi
Navigate to the folder where your extension is located. You can build it in the usual way using web-ext:
web-ext build
You can install this ZIP file permanently in Firefox by going to about:addons and dragging this file into the tab.
In order for this to work, you need to set xpinstall.signatures.required to false in about:config (works only for Nightly and maybe Developer Edition).
You can try setting the preference extensions.legacy.enabled
(this will only work in Nightly or Dev Edition).
You need a "blueish" Firefox -- Developer Edition (effectively beta) or Nightly (unstable, updated every night).
You can get them from https://mozilla.org/firefox/channel/desktop/.
Then xpinstall.signatures.required
will work again.
(As for permissions--you can create a GUI in a tab or a popup, but I don't think you can do it in a separate window (unless you do a webpage-style popup window). You won't be able to write to arbitrary files anywhere on the system--which is a good thing! You can write to the Downloads folder, and read/write some sort of internal storage, but that may not expose the actual files involved. For more information see https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Working_with_files.)
What you should be looking for is having your extension signed by Mozilla as Unlisted.
See Mixing Listed and Unlisted Add-ons on addons.mozilla.org blog post for an overview.
That way, AMO does not host nor (normally) review your extension; it simply runs some basic automated checks and immediately signs your extension so that it can be privately distributed as an XPI.