Error: No Window Matching {"matchesHost":[]} For Tabs.insertCSS()

I'm writing a WebExtension where a background script communicates between a popup window (UI script) and web pages (content script). Using port = browser.runtime.connect({name: 'Pr

Solution 1:

It took me adjusting it for Chrome and looking at the background page there... Although I have logging for extensions enabled in Firefox, it didn't give me what I needed, but Chrome did: It was another missing permission.

Unchecked runtime.lastError while running tabs.insertCSS: Cannot access contents of url "[url]". Extension manifest must request permission to access this host.

adding "<all_urls>" to the permissions in the manifest.json did the trick:

"permissions": [
    "activeTab",
    "tabs",
    "storage",
    "<all_urls>"
],