1. Load the attached WebExtension. 2. Visit example.com 3. Look at the console of the tab (Ctrl-Shift-K). Expected: Will call location.href = 'javascript:console.log("javascript:-URL from content script");' - you should see that message javascript:-URL from content script "This is an injected <script> and will now assign a javascript:-URL" "javascript:-URL from injected <script>" Actual: The second line is missing, which indicates that navigating to javascript:-URL from the content script failed. Attempting to open a javascript:-URL from the page itself succeeds, however (as seen from the last line). Assigning to javascript:-URLs is used by some extensions to run code directly in the page, e.g. if they want to modify some JS variable of the page.

This is intentional: http://searchfox.org/mozilla-central/source/docshell/base/nsDocShell.cpp#1451-1502