Go Private Quickly от Don C.

Here's the Firefox description — plain text, ready to paste:

Go Private Quickly does one small thing and tries to do it well: it puts a button in your toolbar that opens a new private window. Click the icon, click the button, you're private. That's the whole idea.

I built it because I open private windows all day and wanted it to be one click instead of a trip through a menu — and because I wanted something that stayed out of the way and didn't quietly phone home. This one never connects to the internet at all.

WHAT YOU GET
- One click to a new private window, straight from the toolbar.
- A toolbar icon that quietly shows whether the window you're in is private.
- An optional "Hardened" mode that tightens extra privacy settings for the private session. A note for Firefox specifically: Firefox doesn't let an extension confine these settings to a single private session, so on Firefox GPQ reports them as "Unavailable" and leaves your normal settings completely untouched rather than changing them globally. The extra hardening currently applies on Chromium browsers — but the one-click private window works fully on Firefox either way.

WHAT IT HONESTLY DOES NOT DO (I'd rather set expectations than oversell)
- It's not a VPN. Your network, ISP, employer, or school can still see the sites you visit.
- It doesn't hide your IP, block ads or trackers, or make you anonymous.
- It doesn't touch your normal browsing or clear anything.

If you want real anonymity, use Tor. For network privacy, a reputable VPN. For tracker blocking, uBlock Origin or Firefox's built-in protection. GPQ plays nicely alongside all of them — it just gets you into a private window faster.

PRIVACY, FOR REAL
- No data collection. None.
- No analytics, no telemetry, no error reporting.
- Zero network requests — the extension never connects to the internet, period.
- The only permission it requests on Firefox is "storage," used to remember your own settings.
- No third-party code, no CDNs, no remote scripts. It's open source, so you can read every line.

ONE-TIME SETUP
Firefox sensibly won't let an extension switch itself on in private windows. The first time you install, a short welcome page walks you through flipping that one switch. You only do it once.

Open source under the MIT License: https://github.com/DJCastle/browserExtensions
Questions or problems: support@codecraftedapps.com