Why is the background script not loading on Firefox add-on (works on Chrome)?

I'm developing a cross-browser extension which works in Chrome but not in Firefox - the background script is not loading.

I tried console.log in background.js and sending a message to the content script and logging message there.

background.js

browser.action.onClicked.addListener(async function (tab) {
   console.log("clicked on extension icon");
   browser.tabs.sendMessage(tab.id, { text: "toggle_overlay" })
      
});

js/content.js

...

browser.runtime.onMessage.addListener(function (msg, sender, sendResponse) {

   console.log("message received", msg)
   
});

Content script works as expected on all code that's not depended on background.js

Folder structure

manifest.json (had to downgrade to v2 because Firefox doesn't support v3 yet)

{
  "name": "Dev Mode",
  "description": "Dev Mode",
  "version": "0.0.1",
  "manifest_version": 2,
  "icons": {
    "16": "./imgs/icon-16-dark.png",
    "48": "./imgs/icon-48.png",
    "128": "./imgs/icon-128.png"
  },
  "permissions": [
    "activeTab",
    "contextMenus",
    "bookmarks",
    "scripting",
    "storage",
    "<all_urls>"
  ],
  "background": {
    "scripts": ["background.js"],
    "persistent": false // <-- also tried without, same result - background script doesn't lod
  },  
  
  "browser_action": {
    "default_icon": "./imgs/icon-16-dark.png",
    "default_title": "Default Title"
  },
  "commands": {
    "save-page": {
      "suggested_key": {
        "default": "Ctrl+Shift+S",
        "mac": "Command+Shift+S"
      },
      "description": "some description"
    }
  },
  "content_security_policy": "script-src 'self'; object-src 'self'; sandbox allow-scripts; script-src 'self' https://apis.google.com https://www.gstatic.com https://www.googleapis.com https://securetoken.googleapis.com; object-src 'self'",
  "web_accessible_resources": [ "imgs/*.png",  "overlay.html"],
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "/js/content.js"
      ],
      "run_at": "document_end",
      "all_frames": false
    }
  ]
}

I'm testing the Firefox extension with web-ext run to test the extension locally.