This is what it looks like when I try to install Extensions via the Add-ons Manager.

Does it still happen in a new profile? An easy way to test a new profile is to install Developer Edition and see if it happens there or refresh your existing profile. Make sure you stay signed out of your sync account when testing.

1. Yes it does still happen on a new profile, with the same error. 2. Can't install Firefox Developer Edition (see screenshot attached) 3. Refreshed my profile, and the same error occoured.

I'm signed out of Sync since I was never logged in.

2. Can't install Firefox Developer Edition (see screenshot attached)

Used the full installer for Firefox Developer Edition, and I still had the exact same issue.

Are there any active enterprise policies listed in about:policies?

Try installing Firefox from the Microsoft Store and see if you can install extensions there.

Try disabling any third-party antivirus or security software, this may require rebooting into Windows Safe Mode (networking enabled).

1. Nothing in about:policies 2. Unable to install from the Microsoft Store since that's blocked by my workplace. 3. Can't disable any third-party AV since this is a work computer. (Using Trend Micro Apex One)

Is there any way that I could manually install an extension by modifiying files in my profile directory?

After some faffing around I found out how to fix it.

1. Create "C:\Program Files\Mozilla Firefox\distribution\extensions" 2. Copy the extension to that folder and rename it to it's ID 3. Give the "Everyone" group the "Full Control" permission on that extension and the "extensions" folder 4. Open firefox, and go to the "Add-ons" page and enable the extension.

Only reason why I found out that you can put extensions in "C:\Program Files\Mozilla Firefox\distribution\extensions" because of the Parent Process Browser Console spitting out that it's searching for addons in that folder.