I'd like to zoom in and out webpages using Firefox addon but I can't find anything in documentation addon docs. I'd like to make it work as Ctrl + mouse wheel or Ctrl + + / - but it should use my addon. Please help.
ChromeWindow exposes a FullZoom object which allows to programmatically control zoom.
However I don't think you can reach it from the Add-on SDK.
Edit:
In light of Wladimir's answer, I would recommend the following
var {activeBrowserWindow} = require("window-utils");
activeBrowserWindow.FullZoom.enlarge();
activeBrowserWindow.FullZoom.reduce();
activeBrowserWindow.FullZoom.reset();
While you can't set arbitrary zoom factors, the bonus is that you get housekeeping for free, i.e. per site persistance of the zoom factor.
Check FullZoom at mxr