In Firefox, your code works (outputs to console), as it is written in the question.
If you are not seeing it in the console, then you are, probably, looking at the wrong console.
Mozilla describes what extension output can be seen in which console in their Debugging page.
You should be using the Browser Console. You can access it from ToolsWeb DeveloperBrowser Console (keyboard shortcut Ctrl-Shift-J, or Cmd-Shift-J on Mac).
If you have it enabled, you could also use the Browser Toolbox console. You can access it from ToolsWeb DeveloperBrowser Toolbox (keyboard shortcut Ctrl-Alt-Shift-I; On a Mac: Cmd-Alt-Shift-I).
To debug your add-on you can use the Add-on Debugger. You can access it though about:debuggingDebug.
You, probably, are looking at the Web Console (keyboard shortcut F12) which is associated with only a single tab. This is what you want when debugging a webpage, but not an add-on. For a content script which is injected in that tab, the console.log() output will show up in this console. However, you will not see output from any other portion of your add-on (e.g. not content scripts in other tabs, not background scripts, etc.).
Showing the correct console for your extension is a bit more complex in Chrome. Console output will show up in only one of multiple possible places, depending on from what context the console.log() was executed. Each of the following DevTools are independent of each other and are displayed in separate windows, or tabs. Displaying in the associated tab (bottom or side) is the default for the DevTools associated with web pages and content scripts, because those are specific to the tab. For the web page/content script DevTools, you have the option of having it displayed in its own separate window, or docked inside the tab (side or bottom).
As Srujan Reddy explained, you have to go through multiple selections on a drop-down menu, to get to the chrome://extensions page (or you can type that in by hand as the URL, or use a bookmark) then select both a checkbox ("Developer mode") and then click on the "background page" link. Then, you have to select the "Console" tab on the window that pops up.
Output will be shown in the regular web console (in the web Developer Tools). You can open it by pressing F12 (or other shortcuts) in the webpage in which your content script was injected. Each web console will only show the output from the scripts injected in that tab.
Right-click on your browserAction button and select "Inspect Popup". Alternately, right-click within the popup and select "Inspect". Either will open the DevTools for the popup page. The popup will be kept open under more conditions than it normally would, but will still be closed if you switch tabs, etc.
Right-click within the main content of the Options popup (not the title bar) and select "Inspect". This will open the DevTools for the options page.
When the panel or tab is focused, you can open the DevTools by pressing F12 (or other shortcuts), or by opening the context menu (right-click) and selecting "Inspect".
@gal007, What are you not seeing? In which console? I'm seeing popup console.log() output just fine in Firefox Developer Edition v55b3 in the Browser Console, the Browser Toolbox and the Add-on Debugger. In Firefox Nightly v56, I also see popup console.log() output in all three tools. I did experience problems with the Browser Toolbox and Add-on Debugger when I tried to use them simultaneously in multiple instances of Firefox. However, they were broken completely (in all but the first instance started, when trying to use >1 instance of Firefox using those tools), not just for popup output.